Upgraded shields does add about a 20% buff to your shields. Some perks it has...
1: Sniper/Beam Rifles take two shots to kill, either body or headshot (excluding Nornfang/Delta)
2: It does increase the resistance to perfects. The Magnum now takes 6 shots to kill (gunfighter is 7), the BR will instead of taking 11 bullets to kill (3.66 bursts, or 4 trigger pulls) now takes 13 bullets to kill (4.33 bursts, or 5 trigger pulls.) The DMR will in fact remain a 5 shot kill, thanks to the increase in damage from the Magnum (it's not just a level 6 req for it's range). The Carbine I'm not sure about, but I believe it will be bumped up to an 8 shot kill. The unscoped light rifle will be 5 shots to kill with the scoped light rifle being 4 (Dying Star is still 3 shot). Interestingly enough, the Whispered Truth now requires all 6 bullets to hit to land the kill, and the CE magnum now takes 4 shots (I think, I've never tested/seen a test).
3: Splatters are now a lot less likely to kill, especially from careless teammates. combine this with Kinetic bolt weapons and you might manage to prevent more deaths while hijacking warthogs.
4: Upgraded shields actually influence overshields. While the deterioration time of the OS will remain the same, the shield itself actually gets a notable buff of the same percentage. It gives you about 6 extra AR bullets to take before it cracks. Worth noting.
5: This is actually incredibly powerful when combined with Mythic weapons. The Tartarus's Gavel and SPNKr Prime both exhibit improved shields by default, at about the same power of the basic upgraded shields. However, Upgraded shields stacks with these, providing the player with dramatically upgraded shields. At this point I'm guessing the shields have a total of a 60% boost, and can take two sniper rounds before dropping. And remember the Overshield works in tandem with this, so if you ever pick up one of these weapons and have some req points to burn, go ahead and make yourself a tank.
Oh, did I say tank? I almost forgot (I actually didn't) about Jorge's chaingun. This weapon, by itself, already boasts IMMENSE shield strength. BY ITSELF, it takes a Nornfang shot to drop those shields. Then you can add both Upgraded shields and Overshield to that. With some back of the envelope math, upgraded shields gives you 300 PERCENT SHIELDS when equipped with Jorge's Chaingun, and adding an overshield to that gives you 3 layers of shields, which adds up to over 1200 PERCENT SHIELDS (not counting for deterioration).
I think I've said enough.