Well (again from what I have read and seen in interviews) is that when they feel that you can keep up with them and interpolate seemlessly with them in real world ops. Starts out with them cross training or help raise these units in the first place. The SK SMU, GROM, the Israelis, the increment from SAS.
The "Tier system" in the US military is a bit of misnomer that's now basically a marketing tool for games/kit and retired guys selling a service. Originally it was just a line item in the budget to conceal (badly) where the money was going as Delta/DEVGRU were not officially acknowledged
The more useful term is SMU, of which there are now 5 in JSOC /
Understand that their standards are completely different. Their tolerance for error even in training is absolutely zero.
For example From what I have read Delta/DevGru assaults structures far far faster than the vanilla Seals and ODAs. They shoot 300-500 rounds a week when not deployed. Everything from their helo insertion SOPs to shooting profficiency, breaching tactics is far superior to the tier 2s. Far faster and thus far more dangerous which is why they need to learn to shoot fast and shoot accurately.
Breaching is their specialty, it's one of the things they were set up to do as a CT/HR unit...The FBI HRT could take down a structure faster than a white SEAL team, the regular SF CRF/CIF companies were pretty good at it too until thy stared to be disestablished a while back.
To be fair the guys on the black team (CT) rotation would be shooting 300+ rounds per day.average /