I've been (Can Picafort, just up the road) in either the 1st or 2nd week in September for the past five years and it's nothing like early season with respect to cyclists. In fact a couple of times we were charged less at the bike hire place as it was low season. The weather has always been better than when I've been in March and the sea is beautifully warm in September.
Typically the weather in September ranges between 21 and 28 degrees, so for most people (including myself!) that would be hard going!!
You will still see cyclists but not in the same numbers as Winter/Spring time
September's a great time to go, although I'd choose second half, just so it wouldn't be too hot.
The sea is lovely and warm, the roads are empty, everything becomes cheaper - what's not to like?!