Twisties are where the fun lies in the 4/4 Sport, so suspension set up is the best upgrade you can do IMO with a small amount of tuning.
Panhard rod
Traction bars
Brake reaction bars
AVO shocks
Negative camber plates
Front springs (but not Suplex as they are a comfort upgrade not a performance upgrade IMO)
Move the track rod ends to the top of the stub axle.
I agree pretty much completely, and over the last 15 years I've been evolving my 4/4 to be as you describe, the only differences being:
1. The Ecotec Z18XER conversion gives c. 170bhp, which is just right in my view - a bit more than a stock 4/4 Sport.
2. I haven't needed to change the track rod location as I have a steering box, not R&P
3. I haven't fitted negative camber plates yet, I've not noticed any issues with turn-in agility
4. My car is so old it had the old Girling single-piston brake callipers, so these were replaced with twin piston HiSpec jobs from Peter Mulberry.
5. Although I agree the Suplex conversion is more about comfort than performance, I find the additional compliance is very welcome on poor quality roads.
6. My original 4.11:1 diff wore out its crosstree, and rather than having it rebushed I got Billy Bellinger to put in a Quaife ATB. I love it.
7. One point about AVO shocks - despite them being offered in 3 different lengths/movement ranges, I ended up having to get them to make me a custom set for my car to ensure they didn't bottom or top-out at each extent. Fortunately they were able to do this at no extra charge, and it only took them a couple of weeks to do.
The result is a car that goes extremely well, and outperforms my ability or self-preservation instinct - as comprehensively demonstrated a couple of years back by Spanner Juggler, who left me trailing in his wake on the A835
