Depends on their level, but you're only looking at 8 to 11 HP less between levels 20 and 30, and double that amount as a defender.
On most Pokemon, attack and defence are in my opinion less impactful than stamina in actual combat, because the only difference they make in a majority of situations is altering damage dealt/taken from charge moves by a minor amount - usually 1 or 2 HP for 2 or higher bar moves, and about 4-5 HP for 1 bar moves, although that'll depend on the move's base damage.
However, charge moves don't get used enough times before a Pokemon faints to outweigh the extra damage dealt from a Pokemon having more HP, especially if you dodge.
While it does make a minor difference in battle, as an attacker you're probably not going to notice this, because attacking is really easy that a lineup of 6 Pokemon will have plenty of HP to spare even after battling a level 10 gym, more than the HP granted by all of them having 15 stamina IV.
And for defending, you're not going to see the Pokemon being used so who really cares. Attacking is so easy that 10 level 40 Pokemon with perfect movesets and IVs could still be taken down solo by a level 25-30 player.
