The only mod that will increase the dollar value of a sportscar is to stuff the seats with crisp $100 bills. And only if you sew it back up with OE thread.617arg said:
Is it something that would "devalue" the car?
To me it seems like it would be a great way to build an all-motor monster Elise.
If you build it, do it because you want a monster Elise, not because it will fetch high dollars on the resale market. Expect to take a loss, and extract your value in years of fun.
The auto trader is full of cars with "$xx,xxx in upgrades!" can't pass smog, asking $6k obo. Mine among them.