I think they would help venting a 750 when you start increasing the bore 10 or 11mm and stroke by 5mm.
We discussed extra venting in a thread when my engine had some case pressure weeping oil around oil filter cover. 71.25mm pistons pump more air 
I should have done it earlier with my 836 if I knew.
It was enough to add the vent thru the clutch cover as you have Mike.
I think Sam Green mentioned extra top vent thru the tappet covers, IN 2&3.
This solution is really nice then .
To be honest PeWe, I don't think the extra breathers are necessary and the top breather from the cam box is quite adequate.
I remember the original thread well and fitted the extra breathers on numbers 2 and 3 tappet covers of my Auto in the States just to be safe.
In the UK, if you are racing a bike, all breathers must terminate into a catch can and the same was done on my Auto in the States.
When I got involved with the CB750 in the first place, the 836 that I built in the UK had a catch can from just the top breather and the residue in it was monitored after every event. The same was done with the 836 in the USA and even when it was taken out to 915, results were just the same.
When we talk about the pumping action of the pistons creating pressure, is this really true.
When two pistons go down, they create pressure below them (positive pressure) but at the same time, the other two pistons are going up cancelling this out (negative pressure).
Excessive pressure in a motor can only be down to 2 things if it is set up correctly. The first is bore wear and the second is the extra heat generated by the bigger motor and I think like Mike said, this is only in really big motors.
Sam.
