Couple of questions: is your 1M 100% stock ? If negative, does it have any mods which might
eventually impact clutch and transmission (risk of faster than usual wear 'n tear: tune, CDV delete, clutch stop, etc.) ? Have you (regularly) raced your 1M on a race-track ? Did you ever experience a 'shift mistake' significantly straining the transmission (for example during hard acceleration mistakenly shifting from 2nd to 1st gear, instead of to 3rd gear) ? Is your 1M the first car with a manual transmission you ever got to drive ?
Last but not least: how many miles were clocked on the odometer when you took delivery of your 1M + was it by any chance serviced shortly before you started noticing
"a nasty grinding noise when shifting from 1st to 2nd" ?
As was already suggested: get it checked elsewhere. Ain't nothing wrong with getting a second opinion.
Good luck.