Some improvement:

545 ft/s with 8 pumps, 584 ft/s with 12 pumps (6,4 ft/lbs).
I'm not sure what 107's are capable of? And if they're meant to be used with more than 8 pumps.
I think .177 is going to be less efficient in a pump gun anyway?

Here are the adjustments I've made, following the advice I received from DT Fletcher and ped:

1. align blind hole in exhaust valve body perfectly with hole in tube

2. weaker check valve spring

3. pump rod is now "longer", the pump cup leaves a tiny gap for air to be sucked in through the breathing hole.

Not sure which adjustment made the biggest difference: it would have been best to try one at a time, but I feel that disassembling and reassembling the 107 causes a bit of wear, which I want to keep to a minimum of course.

By the way when there's no pressure in the gun, the forearm drops down. So I guess my pump arm adjustment isn't right yet.
Any ideas how to adjust in order to get it right?
Looking at my 108, the pump cup clears the breathing hole more than my current 107 setup. Hmmm.