Do you want a sneak system? Honestly it depends.
How would you want to keep track of it? More on this in the last section...
How do you think it should affect stealths?
There's no denying it, Helovia has gotten really -really- good at stealths. The chances of it actually being unblocked, and passing are incredibly slim. Which honestly is a detractor for me from trying to do one to begin with (not to mention I'm terrible at them). I'm not sure if others feel this (I've tried not to read comments already here), but I feel like we go "nope can't get that" for pretty much everyone. PLUS if you go after the wrong thing you get pummeled. Though fair and realistic it can still be annoying to always have that consequence. So I think it would be nice for the sneaks to get a little heads up with some dice, even considering the expense of my stuff being stolen more. I think to revive the stealth board it does need shaking up. I think we need to give sneaks an incentive for stealthing, something to pep them up. 
I like the idea of experience tying in to give you more of a chance to steal items. Its like the buff system! Which is brilliant. The problem though is how to use it. If a stealth is blocked, and you roll and get the item, would you still have to see if the stealth would pass? Or would the blocking be confirmation enough that the stealth could pass? What if it was just a lucky guess and the stealth was horrible and never could have passed? What then would you do if it wasn't guessed and didn't pass? That stealth should then have an equal chance of rolling the dice. That though completely nullifies the writing portion which keeps stealths in check. And if that goes then you can just write whatever the heck you want. So how will you use the dice yet keep the writing standard. You could like the fighting boards do a mix of the two, but that seriously complicates the process, and you run into the argument that still persists in the fighting boards of whether the writing or dice should count more.
Other thoughts or concerns:
Here's my biggest concern. You're talking about giving advantage to sneaks who are ranked is what it sounds like, and I'm not sure that's a good idea. Now I admit at the moment I could sound rather biased BUT HERE ME OUT. There are a lot of outcasts at the moment who would, if ranked, be sneaks or part sneak.(I'm behind on the times so excuse me) Ophelia, Nymeria, Ghost (was an outcast...can't remember now), Oultik, Aisling. LOTS of those active on the stealth board are outcasts. Now you simply can't tell me they are not skilled/not getting more skilled just they're not in a rank. IN FACT, some of them are probably MORE active because they are outcasts. For instance you can be more crazy sneak without keeping to the ties of herds. So if you give points to those in sneak ranks you completely miss an entire portion of actual sneaks. To compare it warriors, outcasts can still have battle stats and earn buffs even without being ranked. They do get bonuses for being ranked, but its just a cherry on top. Again though, to make a system that sophisticated might be more than you're asking for. 
Additionally, if you're going to have sneaks outcasts or not gain stealth skills, then they will have to be categorized as such. Now if you can do this like battle stats, no problem. But if this is going to be a growing trend of giving each rank systems like this then you'd have an issue. Because characters go in multiple categories and you could have a character with all these multiple stats and it can get a bit out of hand. But you couldn't stop them from having them because it would be very unfair to deny stats to people. 

And just on a final note. Unlike fighting there are a multitude of ways outside the stealth board to show your sneak skills. In fact for those, like me, who are ungifted in the ways of writing stealths this is a major appeal of being a sneak. There are just so many ways to be a good one! I worry then that if we start to put numbers on sneaks like we do fighters they will be judged by that number. Which in the stealth board is fine. BUT what I worry might happen is it might negate any of the out-of-stealth-board sneaks. Again, I sound biased I know but hear me out. A lot of characters have magic, plots, or personality that makes them a better sneak than even some who do just fine on the stealth boards. How can you possibly quantify that? This isn't like fighting where a character's build is easy-ish to translate into numbers. This is about how good a player/character is at moving through the game. 

Finally there is the issue of base stats. You can't tell me that a brand new character just joining Helovia has zero stealth stats. Even brand new fighters get base level stats. It would be unrealistic to say then in the history "this character has been stealing his entire life" and his has no stealth-cred. Of course you could get around this I suppose with the argument that well yes they have been stealthing but they don't know this territory/horses, which is a credible argument I suppose. Yet then again you run into the problem of how can they earn that there you run into my first point in this section. And if you go further with this, and say that, alright the longer you're around or the more posts, the more helovia experience, and this becomes troubling cause you can EASILY get characters who become all powerful in anything. It might be nice to shake it up that way/reward the loyals, but think of how confusing it would be and how turn-off-ish for new players. A new player if they couldn't reach those high levels might be very put off. So then you'd have to max it and it just becomes a nightmare. 


SO basically, I do think sneaks need a level up, something to swing the stealth boards back to their vantage, but I haven't the slightest clue how to do it fairly. Sorry....I fear I'm not much help...