You can still skin at -3 or -4, it just won't be ideal. Same goes for skiing pow on them when they're mounted forward of recommended. I have 185's that are -3 from center and I like them a lot, but it's true that they don't float as well. If you do mount them forward of recommended, don't go any more than 2 cm forward.
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Water Ramps - Rails - Trampoline - Foam Pits - Slip N' Fly