Hmmm. Interesting. I sit in the other camp with regards to valves....surely the inlet is the biggest benefit by far? Getting the exhaust gas out is nowhere near as hard as getting the inlet gas in, for starters exhaust ports flow in the opposite direction to inlets... a large part of what determines the flow efficiency of inlet ports themselves is how well the port guides the air to fully utilise the entire circumference of the valve seat. In nearly all cases this can't be achieved because air travelling at high speed can't get round the short side bend in the port and just skips across the back of the valve head and tries to exit through the long side of the valve seat.
The exhaust port doesn’t fall foul of this…. The air already uses the full circumference of the valve seat by virtue of the fact it's going into the seat from all round the combustion chamber anyway. Provided the port itself is big enough then almost any design of exhaust port will flow at a good efficiency because the valve (seat) is being used effectively. Even the shape of the short side bend is not that critical because it's not having to guide the gas IN to a seat - all it's doing is guiding the gas into the port.
TL : DR- Exhaust valves work better, naturally. Hence inlet improvements being something I would prefer to focus on.
I would be very much interested to hear any rebuttle on the above, seeing as, y'know, I chucked three and a half thousand quid at a cylinder head that you're going to tune for me based on my "inlets are best" theory