My funnel doesn't have magnets, there is a small indented rim that fits inside the portafilter. That would make tamping with the funnel difficult I would think but I never needed to do that even before I started using a leveling tamper that wouldn't work with either style. The magnets are a nice touch. Sometimes I'll bump the funnel a little and some grounds sneak under.
Funnels are a great idea.
The first funnel I orded fitted with a rim to the inside of the portafilter like yours, which meant my tamper wouldn't fit inside, so when removing the funnel lots of the coffee would spill out. That's why I then found the funnel I have now, which has that rim on the outside of the portafilter so that it doesn't obstruct my tamper.