Not that much, at least for raids. Assuming you power up your mons to L32.5, you reach all breakpoints for a 10 atk Ttar vs relevant raid bosses, namely Lati@s and Mewtu. In very few matchups its necessary to have 15 Atk (Entei is the only one I can think of having a L40 bite breakppoint and it may never come back).
Every Atk gives you roughly one more damage per lvl on the charge move tough. Powered a 14 Atk Ttar myself without caring much about that one atk point.