Kaama,
I also have Stellings extension boxes on a 30' Scarab straight Vee. We have approx 780 hp at 6800 RPM. We also borrowed a set of 5B 28P Herrings (from OSO member) as a final test at Lake X. We picked up 2 MPH (107 MPH) turning them in vs. 105 MPH with labbed 28P B1's turning in. We couldn't turn the Herrings but 6600 RPM...really needed 27P. My boat (and me!) did not like the 5B Herrings as it was squirrely due to the stern lift. The Herrings turned out to be closer to a "final" test than we planned for. We actually lost control at 107 MPH...it is nice to still be able to typeThis testing was done in 2002 prior to the 5B Merc props being released.
Our testing was only for top speed and acceleration, stop watch comparison runs from 50 to 80 MPH. We never compared at a steady mid-range.
You might consider trying a 1" spacer to see if you can pick up the mid-range performance without losing much on the top.

Picked up 4 mph so far with imco 2 in shortys still testing props turning 28 b'1s 5100 w/1:36 gears @ 87 mph.