I have the performance controller too and I think it just makes it a better car to drive all round. IMHO it does improve acceleration, not in terms of speed, but in terms of the car feeling more planted / stable (especially when you boot it). As DPM says it allows you to get power down earlier and to maintain it out of corners as well.
This is a subtle mod but a worthwhile one, although opinions obviously will vary. This wouldn't be the first thing on my list and in terms of cost it is relatively expensive (controllers new £500 plus vat). If you can pick one up cheaper second hand, as I did, then this is a no brainer.
Regards,
B.