How much of a difference does 10 att iv make on ttar?

No! Not significant, but it is worse:) Maybe need more dust to level up to breakpoints for raids.

It depends on the defender and the moves.

Against Mewtwo, Tyranitar with 10a IV using Bite hits its last damage breakpoint at level 28.5, so probably missing anything there. Tyranitar with 10a IV using Crunch or Stone Edge is about 1-2 damage behind 15a IV.

https://www.reddit.com/r/TheSilphRoad/comments/6l2msh/raid_damagecalc_find_damage_cutoff_level/

If the other IVs are good, it won't be too bad. Are we talking about 10/14/13 or 10/3/6? (A/D/S)

Further, if it's the best T-Tar you have and will ever get, well, then run with it. If you think you'll get better in a reasonable period of time, don't.