Richlstubbz: I'd been flying with a Saito 91 4 stroke and a 16 x 4W APC with great hover/3D punchout.
I did not like the knife edge at standard 120-130mm cg per plans because it could barely hold and not climb in KE.
Then I tried a 15X6 Zinger and a 14x6 APC, still limited KE but hover/3D still OK. Then I tried a 14X7 APC and it fixed the KE performance.
Just a little extra prop blast made the difference. Still hovers very well, but with less punchout. Climbing in KE is important for 3D so I like the compromise.

Prop ________RPM ___Pitch Speed (theoretical from (pitch/12)a.k.a.feet per rev X rpm X 60 / 5280)
16x4W APC __8800 ___33.3 mph
15X6 Zinger _ 8700 __ 49.4 mph
14X6 APC ___ 9900 __ 56.2 mph
14X7 APC ___ 9100 __ 60.3 mph

I think that it is the pitch speed doing it. The 14X6 didn't load the engine enough so the 14X7 did the trick! I got that hint from an unofficial Saito page (http://saito-engines.info/prop_sizes.html) where the author suggested the hard to find 14X7 as the ultimate prop for a Saito 91.
Since your ST90 liked the 16x4W so much, it may also like the 14x7 if you can find one.... its a bit of an oddball, known as the Pattern prop.
Frankly, I expected good KE, but limited hover- but instead I got BOTH working well.