The Z4 will be suffice with just a TD05, decat and panal filter but it will only get around 280bhp max.
A mappable ECU will get much more (but as you say this when your pockets get lighter)
ALSO
Mapping does make alot of difference and most mappers dont need a rolling road they live map on the roads.
Speak to the likes of:
Andy F
Bob Rawle
Jolly Green Monster
Andrew C
Zen Performance
There all on Here - Somewhere
They will give you some very good advice.
Craig