Do you axis fanboys deal exclusively with absolutes?
2 grens/3 osttruppen and 2 PGs very early or HMG42 and PGs very early would be pretty much IWIN combination, suppress anything, mow it with PGs, rinse and repeat.
PGs biggest problem lies with their usability and difficulty to approach targets frontally without cover(which they shouldn't do, they aren't shock troops), not stats and changing timing to earlier pretty much warrants nerfs to performance.
They also are -NOT- a squad that should open the fight, grens or other unit should and get behind the cover, then PGs should move in.
Take a look at my playercard if you think I'm a faction fanboy.
2 Gren, 2 PGren + Pioneer = (100 (T1) + 200 (T2) + 240*2 + 340*2) = 1460 MP
3 Ostruppen + 2 PGren + Pioneer = (200*3 + 200 + 340*2) = 1380 MP
4 Infantry Section + 1 Universal Carrier + 1 Vickers MG = (260*2 + 280*3) = 1360 MP
UKF starts with a combat capable unit and isn't spending 300 MP on teching straight away, which gives them a strong manpower advantage. I honestly don't think it'd be that different from the Volks vs Penals dynamic: Penals win 1v1, Volks have the numbers advantage.
If anything, it's USF and T2 Sov I'd be concerned about.
I believe that would be too soon. Their deployment at earliest could be around anywhere from 3-5 minutes into the game that doesn't include skipping T1 builds. They already have a number of counters also available before they arrive and at least they have to fight at mid-short range to be effective.
I think everyone's in agreement that if you stuck them in T1 they'd be far too strong.
My thinking on opening with T2 is it's a tradeoff of firepower for field presence. In the fights you take, PGrens are going to be strong: they're a 340 MP squad.
But with an extra 100 MP on teching off the bat and 100 extra MP per PGren squad, you're going to have a much smaller army. If you split them all up, the enemy can defeat you in detail. If you keep them together, you lose a lot of map control.
It'd be a lot like the Soviet T1 opening, just without the highly mobile clown car to drive off enemy capping squads.
Your Headquarters after Battlephase idea could also be very interesting, although ironically there you'd be deploying Panzergrenadiers to skip the light mechanized T2.