Ok, I just checked and there's no real DPR gain, something like 4 damage ahead on your attack and 4 damage behind on your reaction. At least at level 10.
Assuming 50% hit:
Level 5/5 fighter/rogue = (3d8+15+3d6)*50% + 3d6*75% = 27.375 ~ 17.375 + 10
Level 10 rogue = (2d8+10+10d6)*50% = 27 ~ 13.5...