The BS170 (finals) are low power devices with a thermal impedance of 150C/W.? Trying to reduce the internal chip temperature by lowering the already low external thermal impedance is pretty futile.?
(thermal impedances add together just like electrical impedance).
So imagine a bad SWR causing the dissipation in the four finals to dissipate 2W, 0.5W each, then the rise within the transistor will be 75C.? If you find the mounting surface feels painfully hot to touch (cannot keep a finger on it) then it is 44C only a rise of 15C to 20C over the case temperature - a fraction of the transistors internal temperature rise.?
Someone can take the time to do the thermal modeling, its quite simple.
