There's a lad on RSMEGANE.com who's just had this done and reports very good results. Said there was a noticeable improvement in the engine performance and operation.
Can't all these improvements be had with a good service? Better mpg, less noise, better response. I would have thought for the full benefits and to be worthwhile it would have to be a high miler which does short town driving. Just my opinion obviously.
You'd probably see small improvements with a service, terraclean is done via fuelling! I never had mine on a rolling road but a mate of mines done a before and after on the same dyno and it made an extra 10bhp after the terraclean