Its all of about 5mm. Worrying about resale value is a waste of time when there are far more other things to consider which will actually fail, need replacement or repair. You won't even notice the difference unless somebody told you it was done and you undid the nuts!

My car is worth nothing as it is. If anything it will be worth more to the right person if I get it really sorted.

Enthusiasts understand this kind of stuff and if they know exactly what they're buying won't be put off. Or as above, if you're OK with it just don't say anything. Zero chance of anyone spotting drilled turrets.

I have a pair of front GT3 monoball shock mounts if anyone is interested, not new (approx 5,000 miles only ) - Part #996 343 181 9R.
When I fitted my PS9's I decided to stay with the standard rubber bushing top mounts as I have got nearly -1 deg already with my setup.

How much do you want for them?

Bfan - make me an offer and I can then work out the postage to Iceland......