porkypete said:
Trouble is, beefier tyres often have more "tread" pattern... and you end up with less total rubber on the road.
Try a 25mm slick for a little more confidence without it being a "touring" tyre?
25c is probably the largest for your rims. Will give marginally more contact area than a 23c.
Tread is of no value on a road bike (in fact it gives less grip not more).
Slightly lower tyre pressures will give more grip (I always run the front 5psi lower than the rear).
BUT
At the end of the day, a tyres grip is down to the rubber compound in contact with the road and this can vary widely. Softer compounds grip better but wear quicker. My 23c Pro-Race 3's are grippier than a grippy thing, but they'll not last long. My winter 25c Krylions are harder wearing, but less grippy and I hate them in the wet.
If you can afford them, Pro-race 3's are pretty amazing grip-wise in 23c, rain or shine. Light and fast rolling too!