Grants recommendation was that you buy the biggest chargecooler that you can fit in it..and that I need mine to be at least double. David (from chargecooler.co.uk) seems adamant that my setup is sufficient and that I've likely not got it setup properly. He's yet to persuade me though with any real facts - and Grant has all the data he needs from the rolling road, so I'm inclined to believe he's correct.

Bit of a random tangent, but what wheels are you running at the moment? I'm on 195/55R16, but having trouble on one side of the car at the rear where it's rubbing on the bodywork. Not sure if it's best to try and alter the arches (which ideally I don't want to do) or try changing the wheels. Unfortunately I can't run lower profile tyres as the ride height can't go up much more to avoid potholes (I'm almost locked out on the dampers in sag at the moment). If I added 10mm of offset and brought the wheels closer to the car it might work - but I don't know if this will have a large impact on performance. Any ideas?

I'd ask David if you could borrow/try a bigger one. Even to try on the rollers to get some data.

I'm running 195/50/15's on mine. I have a spare set you could try.

I run 195/50/R15 which I calculate has a total diameter of 576mm.
195/55/16 calculates at 620.9mm.
This is a difference of 44.9mm or to put it another way, your wall height from the wheel centreline is 22.45mm more than mine which, depending on ride height/spring/damper settings could be part of why your wheel/s are rubbing.

Also note that there is a 4.4mph speedometer difference if you go to a smaller tyre (if I've worked my maths out correctly).

I can't understand why one side in particular is an issue unless there is a difference in bodywork or the rear is slightly offset (mine is...)

I think I'm running a rideheight of 120mm all round - due to the placement of the on-board suspension pickups I can't get much more height out of it without locking out on the arms (well - dampers, but I set the damper length to avoid locking on the arms!). If I move to your wheel size this will lower the car to 100mm all round which I fear will cause problems on the potholes.

There is a difference in the bodywork left to right - but not a huge amount I can do about it, it's just in the nature of the finish (deviations are small ~10mm or so, but enough to make a difference).

Not quite sure I follow about the rear being slightly offset: do you mean if you measured from the centerline of the chassis (parallel with the direction of travel of the car) that one wheel would be further away than the other?

Also - when you have your camber set up properly, do the tops of the wheels protrude from the arches? If not you may be running more offset than me.

PS. Thank you for the offer of trying out the wheels, but I fear it would lower the car too much...also, Grant unfortunately didn't have a larger chargecooler to hand - and it probably needs something custom to fit.

Just for reference here are a few pictures of the rear wheels.

Never had problems with adjusting the ride height either too low or high (Gaz coilovers). My main issue was setting the height and maintaining a decent level of 'hardness'. Changed to 10.5" 300lb springs recently from Rally Designs and this felt much better.
I think my ride height is about 140mm.

Regarding the rear bodywork, I noticed recently that the drivers side wheel outside edge was flush with the rear arch but the nearside was slightly inset by about 10-15mm. They are both set at 3mm (1°) negative camber and don't protrude beyond the bodywork.

Looking at your photos, have the rear arches been modified at some point as mine have a 1" flat edge which yours don't appear to have...

I have found that the suspension set-up is all about compromise i.e. Hardness, bump steer control, ride height, wheel alignment, wishbone/driveshaft alignment and wheel to arch clearance!

Maybe post some more photos including the spring/damper set-up as it might help our understanding of your problem.

The rear arches have been cut away more yes - to make extra room for the larger wheels. One possible reason I'm running them (I can't remember exactly!) is that the on-board pickups are angled in such a way that to get a decent ride height I need them to be this large. The photo below shows how close the rear bottom suspension arm is to hitting the rail below (there's probably just over 0.5mm clearance, it may even touch before the damper locks out). This is when the ride height is set to ~120mm. It's unfortunately not an ideal setup - I've got 80mm total suspension travel one side, and 55mm the other at the rear due to this issue. I don't really notice is on the road in terms of handling though - all I have is a horrible screech & smell of burning rubber if I hit a hole hard enough as the wheel collides with the bodywork!

In terms of the side that's colliding with the bodywork, the wheel there sticks out ~ 10/15mm clear of the bodywork at the top (second picture attached), the other side is flush. By no means an empirical measurement - if you line yourself up with the chassis and eyeball it, it looks like the wheel is 10/15mm further away from the chassis (the bodywork looks to be symmetrical).