No need for both harder springs or solid lifters on the 20VT.
The oil port on the hydraulic shim is 1.50mm and by decreasing the diameter of the oil port you will decrease the hydraulic valve's tendency to bounch at high revs due to higher oil pressure inside the hydraulic shim.
You can weld the port and then re-drill it.
Matt that sounds like a sweeping statement. I'm not saying you are incorrect as I am not good at fluid dynamics but I would have to advise against that unless you have test evidence supporting this. How do you know how the chosen oil behaves within the tappet? How do you know what size to make the new hole?
IMO if you want to reduce possible valve bounce then you use stiffer springs, lighter components, solid lifters and a milder camshaft which george20VT has done. I still think you will have a hard time getting much past 8500rpm with your setup let alone still making good power but I guess we shall soon find out and will be very interesting to see the dyno plots.
