Sorry for laziness about requoting stuff so neatly but
1. The difference between 30mm and 40mm is huge. Remember, exponential increase in shell volume.
2. Um, yes it will. "Vaguely uncomfortable" is a fairly low marker to aim for anyway, and seeing 40mm tracers go past the canopy is going to make you feel fairly uncomfortable in an aircraft.
3. 40mm just does it a lot better. To the point where it's like an actual explosion instead of just a half-size grenade. And 30mm is far less effective against MTBs and destroyer turrets and armored vehicles.
4. Sadly (maybe not sadly) that is true.
5. Yeah but not with accuracy, so if you miss your firerate goals it won't work well. And it might not work well in general considering that 20mm doesn't hold that much in the way of high explosives.
6. Again, read up on the M42 Duster. The Americans used it (to great effect) in Vietnam. The only problem was overly sensitive contact fuses, which we will not have a problem with. The T-25 was not actually a light tank, it was a full medium/MBT, thus it has enough armor to act as a great light tank even if the armament is a little strange for a light. And if it doesn't have space [again, ex-medium tank should be good enough], well, the only new thing in this revision is the turret. Take the turret out and slap it into something with enough space for the radar.
6.5 As for space for ammo, the Bofors 40mm only ever fired at about 120 RPM and that's at maximum firerate which should be used only to break up massed aircraft attacks, so we shouldn't be running out of ammo since the T-25 had plenty of 50mm ammo, and 40mm is much smaller than 50mm, and it's not fighting tanks. It'll be fighting (like, fighting and not getting annihilated instantly) only glorified tin cans and glass cannons.