The fact that you've made it impossible to roll an 8 by replacing 2d4 with 1d6+1 might impact the game just a bit. Also with 2d4 a 5 is 4x as likely (1+4, 2+3, 3+2, 4+1) as a 2 (1+1) or 8 (4+4); with 1d6+1 all outcomes from 2 to 7 are equally likely, so you're far more likely to get a critical hit or critical miss (if the game has that sort of thing, and you adjust it for the reduced range).