I shot a G35 in Limited for about a year and a half. I put 12k rounds through it and won one club match. I was usually in the top 3 with i . On a whim I bought a Para 16-40, did one practice session, and won the first club match I took it to. Since then I've won 3/4 of the club matches I've been to, shooting against the same people that I shot the glock with. IMO, the glock makes an excellent Production gun, a decent Limited gun, and shouldn't be bothered with in Open.
I know some guys that will switch back and forth from a Production Glock and a Limited STI, and they do pretty well with it. I'd say you just need some more practice with the STI.