The factory manual for the 4/4 states 18 psi for 165/80x15 tyres. My dealer (Berrybrook) said 18-20psi. So 18 psi is what I have inflated them to - cold of course.
The manual does say 22 psi for 195 tyres (presumably that would be wider section on the +4).
I have tried 24 psi and found the steering rather light. I find my suspension reasonably compliant and well-damped, and it doesn't bottom out very often even on rough moors roads.
Surely over-inflated tyres would give less grip?
My car however is only 3 years and 3500 miles old - perhaps you are using higher pressures to compensate for worn suspension?
...also here in the webpage relating to the Morgan Sport GM engine upgrade for older trad Morgans:
http://www.gomog.com/mogwire/review/GM44.html"A brief pre-test check was carried out and I noted new wheels and tyres (Firestone F560) and also brake reaction bars had been fitted during the conversion from CVH to GM 1800cc VVTi motor. I noted also tyre pressures of 26/27 psig which, with Kevin's immediate agreement, I reduced to 19 psig (his preferred figure, mine is 18 psig.)" 
