Whether the 084 or 088 has different published WOT values has little to do with performance - it's just a crude way to get the mixture "about right". Most have limiting coils... and a tach alone won't do it. Best way, if you care about perfection (I don't - just want to be mixture safe) is set it as suggested, then make minor variation with timed cuts.
There's some info in the following thread that I dug up on 08x coils etc.
http://www.arboristsite.com/showthread.php?t=28514
Enclosed is a tech note from Stihl showing the WOT setting of the 088/88 is now 11,500. It's being set at 11,500 just to make sure you don't bounce off the limiter.
On the 084 with a limiter, the process is something like (I don't have the manual in front of me): set the H screw 2 turns left, then turn right until the rev goes up to 10500, then another 1/2 turn. This way they are making sure the limiter doesn't mess with the tach. When done you won't end up with 10,500... it will be more like 11,500 -12k, but you can't measure it.
You MUST be really careful that the saw is in good shape for this method and your air cleaner is clean - never turn it so it's less than 7/8 out..
Why the different approach between the 088 and the 084? the 088/880 has a more precise limiter that gets in the way less.