Bending transistor legs should be OK, especially if you know how to repair them. =) Don't bend them close to the transistor body.
---The following is based on assumed currents involved in operating the power supply at reasonable, maximum levels.--- (I have no knowledge of the actual currents involved)
Your link of images contained the datasheet for the device and it's thermal dissipation rating is 100W with a junction thermal resistance of 1.25 C/W at the junction, pretty typical for a higher MOSFET device operated at the currents involved. I use some Fairchild and Toshiba devices that are in the same ball park. That is a lot of heat to dissipate and that poor little aluminum plate is going to be scorching hot trying to carry away the thermal load.
If I was designing the heatsink for this pair of devices, I'd use a finned heatsink of a much greater size but the poor mfg. has to make sure that the case is user friendly and fits a package size that can be installed into an area that is small. Cutting the side of the case to accommodate a finned heatsink and attaching the devices directly to it would be best if it fits your use. A big heatsink can't just be supported by the device legs so heatsink attachment to the case is best.
Another way to get better dissipation may be to attach the devices to a copper heat spreader instead of the aluminum plate. Something in order of .125" T x 1.5" H x 4" W would be adequate. Copper pulls the heat out faster than aluminum.
About TIM's: The Kapton material sil pads are the best in that lineup posted, especially thin ones. I have some special Japanese mfg.TIM's that are supposedly rated to be best on the planet but gains are infinitesimal. It's hard to beat a thin slice of mica and silicone based thermal paste. Yes, Arctic Silver is better but conductivity can ruin your day.
If a guy is operating these supplies at their maximum, additional cooling efforts may prove to be worthy. For the low price, it may be easier to buy a few and pick the ones that run cooler at higher current. It sometimes happens like that.
FWIW, I found an online hack for modifying ATX computer supplies to deliver 20 A + at 12-24V. I may pursue that as i have a few orphans laying around. If it proves out, I'll post the results.
