You can get 50 flywheel hp (depending on year of car) from headers, 3" exhaust and cat delete. The installation is not bad, I have done both a Gen1 and Gen2. The problem is, without them, all the other mods have limitations. The header/exhaust changeout is the gateway for all the other power adders. Take a look at how peaky the dyno curve is on a viper with other mods and no headers/exhaust.
You can do heads, they will cost you more, give you less/equal hp and are harder/longer to install and they won't work that well without the headers.
As far as the heat, I can only speak for the Belanger brand. I have the double coated ceramic version and the engine compartment runs cooler, even without the heat shields. Removing the cats also solves the issue of the sills and heat.