About the engine selection criterion, OK, it's your car, money, and wishes, so nothing to more to say.
But as I see things, the reason why you have not installed bigger seats for the valves, is that seats are placed very close of the combustion chamber edge, and if you install bigger seats and bigger valves,you should shrink the valves on the combustion chamber, which is not good for flow, or maybe not feasible, so to increase flow you have "open" the valve throat area taking 2mm ID on the intake seat and 3mm ID on the exhaust one, so you have not enough material on the seats to apply a 3rd angle, that is the only reason to justify your 2 angle seats.
Think that what really matters is the HYDRAULIC DIAMETER of the seats, which is lower than the ID of the seats, the 3rd angle helps to adapt the flow direction between the valve seat and the valve, the same as intake bellmouths (air horns) geometry do with intake flow.
- SNO Director, Spain branch, Barcelona
- '90 2.0 Spider -- '93 164 QV -- '93 155 2.0 TS -- '02 156 2.5 V6 -- '?? FMG 3.8 Project