The most TH you can apply with your first action on a mob is TH8, and this will always proc (this includes your gear, traits and gifts). Note that the messages you get about TH in the log are only for *upgrades* to the mob's TH level, which is why it will only comment about TH9 and above (*after* you've already procced TH8, which it won't leave a message about). TH upgrades are random, but you can increase the chance of it happening with SA, TA, higher TH levels of gear, gifts, and merited feint. So yes there is a point in wearing more levels of TH gear than just what it takes to reach the TH8 cap for the first action. However, people have observed limited returns in terms of drop improvements after TH4 or so, so it's up to you if you want to bother. Obviously, your damage output will also be somewhat limited when you wear TH gear, so if the NM is dangerous or you have a time limit to kill, it may not be worth it. Also, TH upgrades can take a white to happen (i.e., people taking 10 minutes to kill Behemoth just to get those last few levels of TH in). A compromise some thfs take is to just wear enough gear for TH8 for the first few hits (especially as you feint and SA and TA, improving the chance at a TH9 message), and then change back to regular gear. It's not impossible for further TH upgrades to happen without wearing all that TH gear, but it will be a lot less common. Again, the question is whether or not you believe those last few TH levels are worth it.