You are projecting more than "a bit" of an increase there. Last year, the top 2 KC RBs (Hunt, West) had 290 rushing attempts and 97 receptions. +50 carries would be +17%. As for the receptions, I have these questions:
- KC attempted 543 passes last season. More or less this season with a rookie QB?
- KC added Watkins and did not lose any significant targets. More or less targets for the RBs?
- Related to the previous questions, is Mahomes skillset one that favors throwing downfield to WRs/TEs or shorter routes to RBs?
In his one game as a starter, he attempted 35 passes, and RBs had 4 receptions. Playing one game with backups is not predictive, but it's the only NFL data point that exists, and his RBs at Texas Tech didn't catch many passes.
I'll take the under on both of your projections (340 carries and 120 catches), although I agree with you that he could reach 300 touches.