I fitted a replica Aero spoiler on mine about 6 months ago and I haven't had any issues with speed bumps or kerbs to be honest as it slips on over the original bumper rather than extending it massively.
The replica which I got from SSD was cheaper than OEM but it is also a few mm out in one or two places in terms of fitment but only noticeable if you really get close down and look. At the time I was getting quoted £400+ for supply and fit of the OEM spoiler which IMO is silly for a bit of plastic.
Oh and do not buy GRP ones, they are shite. Make sure it's polyurethane plastic like the OEM ones.