Ive had it done last week or so. I am on 55lb blacks, T35 and running about 390bhp.
Idle and drivability has improved (mostly due to MSD chip). Driving round town and putting your foot down still uses a lot of fuel but off boost driving it really has massively improved.
On the motorway at 70-80mph it happily hits 35mpg. Big big improvment, and defo recommend it but dont expect miracles if its mainly round town driving you do.