I've got 1 inch slicks on my bike and the difference is incredible. Unfortunately, it's kind of broken atm and I've been borrowing my brothers bike on big 2.1 off road tyres instead. However, checking the Endomondo tracking stats tells me that his bike is - on average - 5-6 minutes slower over 7.5 miles than my bike.Went to the bike shop today and the advise that I can go down to a 1 inch tyre on the rims I have. Had a brief look on the bay and for just over £20 I can get both the tyres and tubes delivered.
I would estimate that difference is:
5% setup (short/high stem & geometry as opposed to long/low setup of mine)