In theory, the longer the C dimension, the slower the head rotates. However, in my experience, with so many other factors involved in getting the club face back to square, a few thousandths of an inch variance in horizontal cg location can not practically be the reason for club faces being left open. The positive aspect of the longer C dimension is the stability of the head through impact. The problem with the majority of modern irons is the blade lengths are too short and the horizontal cg is short, and rarely in the center of the face. Two things happen when the cg is short toward the hosel and not in the center – the head may rotate very slightly quicker, and you will loose distance. For every 1/4″ off center a ball is struck in irons, you loose 5% of your distance. Most misses are towards the toe, so that just makes it worse. Not a fair tradeoff of, in my opinion, by having the club head rotate a few millionths of a second faster with the horizontal cg shorter and towards the heal. Our testing, even with the really extremely long horizontal cg’s that Ralph designed in the past, was that golfers hit them straighter, closer to the correct distance and much less offline (especially on off center hits), than designs with shorter horizontal cg locations. We have moderated the designs a bit away from the extremely long horizontal cg locations (mainly because of the look), but we do maximize it in the designs we do, creating the most stable and playable irons for whatever category the design falls into. Stability is still the key factor and you can’t get it with shorter horizontal cg’s.
I would give the STi’s a little more time and maybe even have a qualified professional take a look at your swing. I know we never think it could be something we are doing as the player, but again in my experience as a player and a fitter, it is more times than not. If the shaft and length and the lie (and everything else) is fit properly to you, and your alignment, set-up and swing are in tune, I do not believe there is anything in the head design that can be causing what you have experienced with the test iron.
Hope this helps,