I think I'd tip over backwards! There's a very steep (but extremely short) part of the Hudson Greenway near the GWB where on my folder I have to conciously lean weight on the front wheel or risk it coming off the ground if I stay seated. Even if it were 30% it's nothing like 300m of it! On the road bike I normally get out of the saddle (not enough gears to comfortably stay seated there) and haven't felt the front wheel jump so I'd say it's probably a good deal under 30%.