Another update. Andrew has introduced a thread priority update as discussed here in commit: a97ae1c7a73ddcdbda0e2ece0fb7c787ab79426f.
This appears to have bought a little more for 2 and 3 threads (again with max fps set at 25):
Max VCO1s (1 thread): 14 (102%)
Max VCO1s (2 threads): 26 (196%)
Max VCO1s (3 threads): 29 (292%)
Max VCO1s (4 threads): 28 (389%)