25mm tyres

Dude, by all means! But graduate off your 23mm at 140psi and try 25mm tyres at 90psi for a week and I'll eat my hat if you don't go wow, that was a revalation ;-)

I notice the drop in speed/increase in perceived effort when my tyre pressure drops from 120-100psi from a few days of riding on shitty London roads, the difference is huge, which is why I top my tyres up every other day, lower pressures increase tyre wear as well since you are increasing drag and the tyres footprint on the road, I can't even begin to think how shit it would feel at 80-90.....no thankyou!

For the record I currently run 23C tyres front and back, but I think my 28C's I had way back earlier this year, were faster at the same pressure, from my experience I think 28's are probably the optimum size for speed/comfort through the streets of London.
Having tried every extreme either side of this size, I can honestly say that 28'c felt best, and thats what I will be going back to shortly.