Rich,
Walker's did up a Shindaiwa 488 for me this Spring. It was a nice moderate gain in hp (maybe 20% or so, they forgot to do a baseline dyno run) but a very noticeable difference in smoothness and throttle response. It is still picking up hp I'd imagine, as I haven't used it a ton and Shindaiwas seem slow to break in. It runs more like our 036 than my other 488. Very well worth the money, but it is not quite the screamer a Walkerized 346xp is.
Walker's also modified the muffler for my 377 for a very reasonable amount and that really made that saw come alive!