I tried the Sub purchase on G1 once, it worked. However from experience, unless you molest all their destroyers, they can just use their remaining navy and the fighters on the UK to kill off your subs. It buys you time, but not a whole lot as the threat of Sealion is gone and the UK will go on the offensive. Sealion purchase on G1, G2 gives you two things:
Two defensive purchases by UK on UK1 and UK2 meaning it will take the UK multiple rounds before massing a force big enough to harass Holland and Normandy.
Flexibility on G3 to either go ahead with a Sealion on the UK or feign and send all those troops into Leningrad.
I generally try to play defensive on the southern front of Germany (Ukraine area) while moving my stacks of infantry and tanks north towards Leningrad. By taking Baltic and Poland on G3, you can mass them all up on G4 with your surviving units from Leningrad in Belarus which puts you on Moscow’s doorstep by G5 and G6 going after Moscow.
If for some reason the Russian mass the Eastern Front at Baltic, you can also land your units there if your odds don’t look good (but I think they should) or you can split them to take out a weakened Leningrad.
Basically going Sealion G1/G2 gives you the duel threat and forces at a minimum a defensive purchase for multiple rounds for the UK without having to “waste” resources (Trn instead of Subs) to do it. You can also SBR the UK on G2 and G3 to further hinder its resources to change gears to attack mode.