The single limiting factor on a standard YB head is the SHOCKINGLY poor valve seat profile and throat shape.

My stage 1 head for example resolves this issue but maintains standard port sizes and can be used to create a comfortable 420bhp with a T4. 450bhp is about the max I go with this spec head.

Stage 2 sees an increase in port sizes to match the revised valve seats and throats and can be used in engines up to 600bhp. Essentially this head has extra flow potential due to the increased port size.

Stage 3 is taking the stage 2 head to the max port size on both inlet and exhaust and is good for 650bhp.

Stage 4 is my latest evolution of head and utilised the stage 3 porting, but I have had the std cosworth valves remade in to std specification but with 1.5mm oversize heads. These heads are good for 750+bhp.

Hence in summary the std head is choking up beyond around 360bhp but will struggle on to 400bhp with the right turbo. Reprofiling the valve seats and throat area can achieve up to 450bhp. After this power level enalrging the ports is necessary but the degree to which this is done is dependant upon the rest of the engine spec, rpm band etc etc.