This is how it should be IMO.
(Round numbers done for simplicity.)
LR AWD - $48,000
LR AWD - After Boost upgrade - $50,000
Stealth Performance - $52,000
Performance - $54,000
So make the stealth upgrade $2,000, the full P acceleration be $4,000 more than the LR AWD, and the full on Performance w/ 20's, etc. be $6,000 more than LR AWD, $4,000 more the LR AWD with stealth, and $2,000 more than just the Stealth Performance.
Also they should make a Stealth Performance accel unlock for LR AWD owners for $4,000 so you aren't stepping on the full out Performance owner's toes.