I've been using the same brand magnets (different designs) for 5-6 years on different cars. The only time they don't hold up is during heavy rain (or during a street survival instruction when you're getting blasted with water by a firetruck), so you gotta use tape. I have a set of "professional" magnets as well, but they don't match the car, which is what prompted the homemade LCD design:
I'll probably get something like that made for the BRZ as well (once the C to E class debacle is solved), but the first purchase will be proper wheels/tires for next season. I'm thinking if the stock road tire class becomes permanent, I'll compete there. If not, then I'm not sure.