I also had heat issues under the passenger seat and along the transmission tunnel. When I replaced the carpets a few months ago I took the opportunity to strip everything right down to
the metal floor panels. First laid down "Thermo-Tec Acoustical & Heat Shield", ontop of which I placed "Thermo-Tec Cool It Insulating Mats" and then laid the new carpets on top of that.
I did the whole car including the rear panel behind the seats and the trunk.
Problem solved, total cost was about $800. for all the insulation material.
Best,
Mike