There won't be much in it, imo.

Acceleration wise my Grandad's Maestro was booked at 12 seconds to dash to 60, this is the same 1275 A series engine the Mini will have, his was actually A plus but it's essentially the same engine, so with your modded head, means more power and more power equates to better performance

To add I find multivalve engines to deliver there goods at the top end, whereas older 8V engine tend to favor low down lovely torque

Martin