If you have the seat pushed all the way forward and the fit guy says you need a larger frame (and you have a short stem), something's wrong. pushing the seat forward will effectively shorten your reach - the opposite effect of a bigger frame with a longer top tube. maybe i'm misunderstanding the meaning of "pushing the seat all the way forward".
if you want to get your knees further forward so they're closer to being over the pedal spindle, and you already have your saddle pushed all the way forward, getting a larger frame like the bike shop person recommends will almost certainly make the problem worse - pushing your seat position further back relative to the bottom bracket. if what you're saying is accurate (and i'm not misunderstanding) then the expertise of the bike shop employee might be questionable.
It just so happens that I saw a Fizik Arione come up on ebay and the seller gave the fizik specs which says it has 85mm of rail adjustability - the longest in it's category. you could measure your longest rail and see how much of a difference it might make. this is assuming that your seated position is the same relative the the rails on each saddle and the extra rail lenght is in the rear of the fizik - a big assumption i guess.