How much of a noticeable difference is there between an 80IV and a 90IV pokemon?

80% IV means a pokemon's total individual stats equal 80% of (15+15+15) ~ 36, so 90% IV ~ 40.5. Each value is then plused to that pokemon base stats to calculate CP & HP. So a Dragonite or Snorlax with above 90%IV is awesome but if his IV is only 60-70 he is still way better than a 100%IV Nidoking or something else.

I'd rather have an 80% with the best move set than a 90% with the weakest.

No noticeable difference. VERY little cp loss, that's it.

You wouldn't really feel the difference attacking but, it can be like 20-30 CP diference that make it over some mon in a defensive gym.
Sometimes more IV doesn't mean more CP this is because Attack is more considered in CP equation

Here an example with Slowbros all Lvl 30

15/10/12 IV 82.2% CP 2183 HP 147
12/12/13 IV 82.2% CP 2166 HP 148
13/13/15 IV 91.1% CP 2193 HP 149

As you can see, there is more difference between the 2 with 82.2 than the 91.1 and the more Attacker of the 82.2

You can use the IV calculator of pokeassist to simulate evolutions and higher Levels to see future CPs

PSA: So many "pro" players are taking IV way to seriously.

Some people go on here and ask if their 86% IV or their 86,5% IV Dratini will be better after evolving. Of course you can strive for perfection and aim to only evolve over 70, or 80%. The CP does not change that much and due to gym stagnation it does not matter anyway if your 'mon is on stage 4 or 7.

The only time such a minuscule difference matters is if you're vying for a high slot in a gym: in a gym full of Vaporeons, for instance, an extra 30 CP might move your Vaporeon up 2 or 3 slots, making it somewhat safer from being knocked out. In terms of battle performance though, no way you'll notice a 10% difference in IVs.