I have now done two TW225 6-speed conversions. The first was TTR225 engine with a small overbore to 226cc with a 10.25 Wiseco piston and a stock TW carb.
The second was an XT225 engine with no engine mods (aside from Placerlodes's shaft extension) and a stock TW carb.
Of these two bikes, the first one with the higher compression piston clearly has noticeably more power, at least according to my butt dyno. Although they say there is no replacement for displacement, I feel that the higher compression piston will give you much more bang than an additional 25cc's at the stock compression ratio.