Despite having a lot of work for the past I've taken my first pass at using prediction to determine what the values should be for Quantum Beam (IMP damage, ignores all armor). The results were disappointing. I'll post the numbers later but the up-shot is that using standard Power Blast (Ranged IMP), Burn (Ranged IMP, hits on the second round for same damage +4 DM), and Electricity (Ranged IMP, -1 CON roll) as bench-marks did not correctly predict what the values for Q-Beam should be.
They were not far off but they were far enough that it would be wrong for purposes of the game (in fact, Q-Beam's damage values came out higher than the predicted values: something I didn't expect).
This means that just "using math" instead of testing won't work (well, it will, but I'd need to do considerably more complex math than just taking a standard modifier for '8 REA Long Action' and applying it to any attack.
That's okay: I can empirically test everything--and I plan to. It's also true that Q-Beam might be a heck of an out-lier (consider that against targets without any armor it is exactly like Power Blast--something that is emphatically not true for Burn and Electricity). I just don't know yet.