60C or 75C wiring for 60C/75C breaker & output heatsink
The idea of modifying the breaker so you could use smaller wire is wrong on a number of levels but I'll just discuss one.  


  But what if you install a big heatsink just outside the breaker. Then you can use 75C or even 90C already?  


The electrical code requires that most of your electrical system be listed by a nationally recognized testing laboratory (NRTL) such as Underwriter's Laboratories (UL).  

The electrical code requires that listed equipment must be utilized according to the manufacturer's instructions;  not doing so violates the listing.  If you modify a listed device, it's effectively no longer listed.  

Adding a heat sink to a breaker would be a modification, and as such would not be permitted.