Without a doubt, if he can afford it , its gotta be the full system.
I have ridden an 04 650 with just a can followed by the same bike will full system,
Can only= not very much of an improvement although sounds like you're going quicker, revs slightly quicker
Full system = very noticable increase in power right through but mainly at the higher end of the rev range.
No dyno results I'm afraid, just using the ''seat of the pants'' mode.
The Akro can make a 650 quite a snappy bugger, so be prepared for that. I preferred the FP power (more power but more linear with it) but its all down to personal preference