The G-Box looked interesting to me but a lack of support forums etc made me nervous. The XBMC Android team developed based on the Pivos Xios DS unit. It is $114 from either Newegg or Amazon (Newegg has a free remote bundle going on at the moment). Supports ICS and probably more usefully a Linux build of XBMC that it can boot straight into.
Not as powerful as the G-Box but very active forums, close links to the XBMC team (Davilla pops up on their forums) made the $25 extra worth it for me. The fact that I believe Pivos also did some sponsorship of the XBMC development was also something I thought should be rewarded.
Caveat - Only had the box a couple of days. For me it seems to do a better job than my ATV2 XBMC unit even though it is still a beta build (Linux). My situation is LAN connections, 1080P out and all media served from a local UNRAID. I'm using the stock skin. YMMV
I suspect though if you are looking for much better 'performance' than the ATV2 (I assume you are talking about skins/navigation etc rather than playback issues) you are going to need to up your budget. The Acer Revo RL7 would be a good machine at around $250-$300. Run openelc or the XBMCbuntu and the thing will fly - hi-def audio as well.
Like most things in life - you pays your money and you makes your choices.......
Tolax
BTW - An ATV2 is typically around $250. Of course the day the ATV3 gets jailbroken that will change