If worry about the heat, get bigger coolers. And to add on to the power, CF driveshaft is the way to go

 

He called you a troll. smFH....

 

The OP also said he wants to keep it quiet, guess what the car will sound like with headers and a catless exhaust?

Your best bet is a V8 engine swap. Throw in an LS with its its OEM exhaust. Problem solved. Besides, your convertible is heavy as ****, you will gain very little torque with bolt ons and barely feel these gains unless you're at redline.

 

Since this most likely can't be done without FI or a engine swap I say open up the breathing+tune and do gear swap (4.08) and lighter driveshaft (CF Or ALUM). Will at least make the car feel closer to 400! Much more fun to drive!