Well, it depends on the bitrate - WAVs are approximately 10x the size of 128kbps MP3s.
Is it really possible to play WAVs on slow Amigas without any problems? I'm quite surprised that decoding MP3 would add that much overhead. But if so, then it's great, I have all my audio in FLAC anyway and that's comparable to WAV in size. The SQ would suck for serious music listening, but e.g. audio books or basically any spoken word should be fine.
