You won't have to rejet with the Triumph offroad pipes, but your bike will probably run better if you do. For that matter, it would probably run better with the original pipes if you rejet. These bikes are setup to run extremely lean to make the various governments happy.
I doubt you will gain much with the Triumph pipes by removing the snorkles, but it's free, so give it a try. With more open pipes, ones that you have to rejet if there is to be any hope of the engine running with them, you can gain up to 5 HP by removing the rubber bits.
The whole problems is that there is little point in letting more air into the engine unless you have provided a means for it to get out the other end. While the TOR pipes are more open than the stock ones they are still a bit restrictive, which is why you can use them without rejetting.
