Thanks for your response Oblivionlord. My bike came with Giant's
PR-2 wheelset and
PR-3 700x25c tyres which are ok on the flat and downhill, but seem to bog down a bit when peddling uphill. As a comparison, I rode a friends Felt 75 road bike quite recently with Mavic cxp22s. His bike seemed to accelerate that bit quicker than mine, most noticeably uphill (there are quite alot of hills around here). Also seemed to run a bit smoother as well, even though he was running on 23mm tyres, though not sure how big a part the frame played in that.
I'll probably upgrade to a set of Mavic Kysyrium equipes when I get some spare cash as they seem to have had quite good reviews.