From an insurance perspective, look to recoup 20% at most up to roughly $5000 (if you have enough additional equipment coverage).
If someone did $15K in mods (parts prices, not labor), and then you deduct the value of the original parts, you might walk away with an extra $1 to $2K like you speculated.
You would also need to supply documentation for the parts.
From a resale perspective, it really depends upon the buyer. I've seen some people get lucky, and get more than they should have, and others that needed to turn the car quick, and lose out big time.