Eh, just some tritium sights and freedom art fibre optic front sights along with unnecessary CNC aluminium slides and outer barrels. Some replacement steel parts like slide catches etc. Nothing major (can be seen here http://shop.ehobbyasia.com/catalogsearch/result/index/?cat=&limit=80&q=HK45)
Tbh nothing there is necessary for the HK45. TM guns are made worse by adding things like metal slides, the whole point of TM's plastic slide is to increase gas efficiency. That's the entire reason why the TM M1911 performs far better than it's WE counterpart, as it is more gas efficient with the same amount of gas as the WE mags.
If you'd read any of the replies in this thread, a lot of them advise against any internal upgrades or metal slide kits. TM guns perform perfectly well, the company has a reputation for extremely good GBB pistols.
As for aftermarket support- the gun came out a year ago and what you see at retailers today are probably the only parts which will ever be available for it. The HK45 is quite a niche pistol compared to Glocks, P226/8s etc, hence why not a lot of customisation parts are around.
It's like comparing something like an AUG to an M4, there's loads of aftermarket parts for M4s as they are immensely popular both in the airsoft world and in shooting circles. Almost every manufacturer makes a line of M4 type AEGs/GBBRs. There's loads of bits and pieces for M4s and such because they are so popular, there's even a place in the US which makes custom CNC hand guards for M4s.
In comparison, AUGs have almost no aftermarket support (and any customisation parts are based off very old designs and almost never in stock) and this is because they aren't as popular, and only a few manufacturers make them.