Trunk mount intercooler

Depends on how you do it and WHY you need to do it based on dyno results and mods.
Throw a cheap intercooler in the boot, add some eBay pipes and it's cheaply done at home for less than $400.
Done properly, you add a sunken floor pan that vents air out the back of the car under the bumper and protects from heat of the exhaust, you run air ducts from making an inlet in the wheel arch side of the rubber shrouding (the part that ducts air into the engine bay) on bothe sides and into the boot, water resist cover your electrical / ecu, spal fans on the intercooler, wiring, small cuts / holes and rubber grommets for piping in and out of the engine bay, custom intake piping, heat shielding.... And the machinist / mechanical shop who does the above gives you a bill for $3000 -$4000 plus :)

In the hot stat of QLD :) Different dyno results have been achieved; depending on your mods.
Bigger piping and bigger intercooler does not equal more power as you need to calculate air efficiencies.
The above exercise of boot mount intercooler does not achieve more power if your intake temperatures are already normalised from standard boost levels (14psi or below) and with stock / upgraded heat shielding and cooling measures. Unless your vehicle is highly modified, a boot mount will decrease response and power.
IMO if your running less than 18psi with full fuel mods, after market cams, air intake and exhaust customised aftermarket manifolds, stand alone ecu, pushing more than a dyno 300 hp at the wheels; less than that then boot mount is not the best option $$$ vs result and possibly a negative return in response and power.

However everyone's build and opinion is individual.