Based on the Power Tune book by Peter Burgess where he displays the results of the power outputs from different MGB engines on the dynamometer.
The MGB 18G engines have a power output at the wheels of 42bhp at 3000rpm
The MGB 18V engines with the big valve cylinder head still has power output of 42bhp at 3000rpm.
MGB 18V engines from 1974 that reverts back to the smaller valve cylinder heads gave a power output of 44bhp at 3000rpm – a small gain of 2bhp!!
But at a higher rev range to give the maximum power output from all engines at about 5000rpm is 64bhp but 66bhp for the big valve cylinder head engines.
It seems that the big valve head produces 2bhp power in the higher rev range where as the later 18V engines has more power (2bhp) in the mid range revs ……… swings and roundabouts?
For my style of driving i reckon the smaller valve head is better as i rarely rev the engine more than 3000 revs. Although if you are wanting more performance at higher revs the larger valve head is better.
I suppose gaining an extra 2bhp is not a lot for the hassle on its own …..but as part of several other smaller gain improvements will give an overall good worthwhile gain in power.