Frankly , all I want to do is get the car sorted. Others have done the roll centre mod and they seem fine.
Well, at risk of repeating myself, I can absolutely guarantee
two things will result from the suggested modifications to the front geometry, if they are done in isolation:
- You will change the caster angle (hence the mechanical trail).
- You will introduce (or change the amount of) bump steer.
Both can be corrected by means of further modifications (ie. correcting the bump steer by means of changing the rack geometry and re-setting the caster by whatever means), but if you've made the roll centre modifications in isolation, then you will
have affected the steering feel, accuracy and stability.
Sorry, but there's no soft-and-fluffy way to put it; we're talking about basic, fundamental geometry here - if you change one thing in isolation it will
impact on other factors.
You will also have altered the weight transfer characteristics (but then that's the aim
of the modification...) and the camber gain geometry (a relatively minor consideration in comparison to the bump steer and caster issues). The new bushes might also muddy the water by changing the amount of toe deflection you're experiencing, too.
Your best bet is to take the car to someone who knows what they're doing(eg. Steve Guglielmi)and get them to correct the bump steer and re-check the rest of the geometry as a start; you can decide whether the caster has had an unacceptably adverse effect afterwards, maybe (the effect of increasing the height of the front upright - ie. the distance between the wishbone outer pickups - will be to reduce the caster, which will reduce steering feel/weight and self-centring to some extent, but lifting the top ball joint by 6mm will only change the setting by a fraction of a degree and you might find that you can live with this).
I have also heard good things of Neil Garner up at Kemble Airfield, nearer to you, but I've never used him personally so can't really give a recommendation. I live within spitting distance of you, so I'd have offered to help myself, but I don't have the necessary gear for checking bump steer or corner weights these days - I was using it so infrequently that I sold it off some time ago. Contrary to the advice on the Ginetta website, you need a bit more accuracy than getting your mate to jack the wheel up and down while you check for bump steer visually, incidentally!