I run 3 elements with my 3X expanders on a couple NUBM44's both at 4.5a and a NUBM08 @ 4 amps, decanned of course and the clipped energy goes mostly into the lens barrel around the lens, I don't see it hurting anything and I get these out and burn daily, I'm hooked on it, so far they have taken hours of this with no ill effect detected, but I do run large heat sinks and I do get them plenty warm.
In short I would not be afraid to use a 3 element or G2 on the NUBM44, just keep the outside covered when not in use or you could burn dust onto the exit of the lens.
With my 3X expanders the 3 elements are sealed up from dust. The far right has a G2,6X pair and 3X expander, they are lots of fun, my point is don't be afraid to work them, just pay attention to your duty cycle, for me it's when they get past luke warm and start to get warm enough that you can feel the heat sinking into your fingers, then touch your fingers to your face and feel the heat in your fingers on your face, but not too hot to touch.
Yes these are ugly clunks, but they are burning fun and I work them hard, and they soak it up and keep on burning, I think big heat sinks give the user more time to decide when its warm enough, also you get to play a lot longer.
ALWAYS PLAY SAFE AND WEAR PROPER LASER SAFETY GLASSES.
Here's an old video of a NUBM44 with a DTR G2 and 6X cyl pair, the power is there and it still works.