No, in theory, a CD Quality WAV file will always be better quality than an MP3, mainly because mp3's are compressed, whereas wav is uncompressed.
CD Quality WAV = 1411.2kbps (44.1KHz x 16 bits per sample x 2 for stereo).
On average, these WAV files work out at approx 10mb per minute.
In MP3, it's the bitrate which gets compressed, and the highest bitrate for 'standard' mp3 encoding is 320kbps. The compression takes out the high and low frequencies not normally heard by the human ear. The lower the bitrate, the less the quality of the mp3, and also the less the overall filesize.
A 128kbps MP3 works out at approx 1mb per minute (~tenth the size of WAV).
To attain a better quality mp3, it's wiser to use the Lame Encoder.
Winamp's DiskWriter output can only use whatever codecs are installed on your system, therefore it uses the Windows default Fraunhofer Advanced MPEG Layer-3 codec (L3CODECA.ACM).
You can download the professional version of this codec (L3CODECP.ACM) if you wish (search google for "radium mp3 codec"), but still it will only support encoding at a constant bitrate (cbr).
CBR isn't too bad at higher bitrates, eg. 192, 224, 256, 320,
but encoding at VBR (variable bitrate) with the Lame Encoder is the way to go for the best possible quality mp3's.
output plugin uses (and comes complete with) Lame_Enc.dll
So you could try experimenting with using vbr
(eg. min bitrate @ 112, max @ 256 . . . or whatever)
Try using both DiskWriter cbr & out_enc vbr, and compare the output files for quality.
Though, if you intend to burn the output files to cd-r, then (in theory) you'll always get better results from converting to WAV than encoding to MP3.
Alas, I still don't know if any of this will work with converting spc to wav/mp3, because I don't have any spc files to play with . . .
Hope this helps in some way (also hope I explained things clearly enough . . . and accurately enough too, hehe, gulp)