Nobody mentioned that the way samples are used as sound sources for sample-playback synths is to multi-sample the source. For example, a sample a middle C, another sample some number of steps away, etc. The sound patch contructed this way only has to pitch single samples up or down a bit, so the speed effect (and all the others, too - the timbre and everything about most instruments can change pretty dramatically throughout its range, so even if the speed-up factor wasn't noticeable, it still would not sound like a real instrument) is not much of an issue.