If you really want larger valves, you will need to have custom valves made with the larger head size. You will also need the heads machined with larger seat insert pockets, and locate new seat insert blanks to use (this would probably be the easiest part, even though you need to pick the correct material). You will also need to increase the spring rates to accomodate the increased mass of the valves. Since you won't be able to run complex valve studies, its better to err on the conservative side with much stiffer. Unfortunately, no stiffer valves exist that the regular 3L springs, and no the ST220 springs are not actually stiffer, I've personall spoken with an engineer from the OEM spring supplier that told me the rates are the same for all the 3L spring part numbers. Basically this means you will need custom wound springs as well.
This all assumes there is enough room, center to center to actually fit both valves on the intake side. Don't forget to account for the seat inserts and the pockets to accomodate them.
I guess what I'm saying is that it could be done if you REALLY wanted to; but I don't think the benefit is there. When using the 3L valves, they have never been the limit for making more power NA or boosted with the D25/D30 engine family. Your $$ and effort is better spent elsewhere.
