There shouldn't be any downside to using a higher picmip setting on modern systems.
Modern GPUs have a LOT more VRAM (Video-RAM) than old pc's used to have. Usually multiple GBs compared to a couple of MBs (when tf2 launched the current high-end nvidia gpu had around 512MBs).
As long as you don't fill up the VRAM you shouldn't see any performance issues and even if you do fill it up they will probably be minimal (there's a lot more to it than just VRAM being full or not...) . Things could in theory take a tiny bit longer to load though.
From my very simple testing tf2 used up around 500MBs of VRAM with picmip 2, 1 GB with -1 and around 1.2GBs with -10 when running the benchmark1 demo.
Prolly because there doesn't always exist a higher resolution image for tf2 to load.
I'm simplifying a lot of things but that's pretty much the gist of it