it will be interesting later this month for me when im doing another rolling road day with my lad and MK3AstraOC. The last one i did i'd fitted sport cats, poweflow exhaust, pipercross air filter and came out with 185.7bhp. The guy running it told me i could attribute most of the power gain to the cats and i left a happy bunny. Since then ive done 4 bar regulator, Magnecor leads, 3.2 plugs, stage 7 motorsport chip, modified throttle body and the car feels much quicker and much more responsive. My reasoning behind the RR day is that i want to know what i'm at now and then do the G cam mod and 2.6/3.2 exhaust manifolds mod then do another RR to see where these will take me to.
Whilst i appreciate the comments regarding going for a 3.0/3.2 over the 2.5 for the power there is the downside that the larger engines in my past experience drink far more. I use mine for commuting to and from work on A and B roads and still return 29-30 to the gallon. When i did the same in the 3.0 i was lucky to see 24-25 and i was doing the same journey driving just the same.
I'll put my rolling road results up when we've been later in the month.