As far as Mod list decreasing the value, this is very misunderstood. To a dealer a large mod list, aftermarket tuning, turbo ect this decreases value to a dealer who would have to imply reliability and some type of a warranty in most states. But on that note let me pose the question.
When you buy a race car with a $10,000 motor, stripped out with a roll cage do you use KBB?
If the work was done well, by a verifiable and reputable shop the car is worth the value that a buyer would pay for it. If you wanted to buy a car from ESX tuning you would pay $45,000 as opposed to its blue book value of $31,000 new. But for example let me show you what I would ask for my car.
2005 Legacy GT 71,000 miles, VF 52 turbo, DW265lph fuel pump, Perrin top mount intercooler, Perrin BOV, Perrin dual cat back, Perrin uppipe, Perrin catch can, Perrin inlet pipe, Perrin ram air, act clutch, act flywheel, lightweight Rims, 225 45 17 tire upgrade and ion lowering springs. Car on mustang dyno pulled 320 HP and 308 lbft at 5200 rpm. 4000 miles on newly rebuilt engine and heads (all from a 2011 wrx).
I paid $11,000 For the car, as it sits now I would ask $20,000.
Now before every one freaks out, know that blue book is about $15k. Add in the cost of parts, figure the demand of the end result. Are there a lot of people looking for fast, reliable subaru's ? Is the car at 100 % visually? What other makes,
models would it compare to? And the buyer of a legacy GT limited looking to do this swap what would the total investment for them to have if they paid full blue book value and then modified from there?
All of that taken into consideration the potential buyer can get in and drive my car and see the results 1st hand, as well as test the motor and check out the quality ahead of time. So is a bolt on car worth $15k if blue book says $9k? Well if the soon to be owner would have spent time with out the car, install, tuning, repair and he/she believes it is, then yes. The only way it would not be is if you were marketing the car to dealers or people in search of well maintained, stock legacy GT's. So is $15k worth it? Check it out, drive it and then add up what others on the market would cost you to buy then modify. Good luck