I had a E4300 before which I ran stably at 2.9ghz. Sure you can get better overclocks on full size systems but medium level overclocks can be achieved quite stably on Shuttles as long as you keep to 2gb. I have always had an overclock Core2 in my Shuttle. Only use 4gb if you do NOT want to overclock.
Anyway this is only my personal experience of having used Shuttles for the last few years.
I had an XP2800(non-mobile) in previous Shuttle which ran at XP3200 speeds daily.