i have a 528, and while i can't speak to jim's mod, i can speak to the fact that i like it on vocals, amps, kick drum, snare, and bass DI. the compressor is nice and the eq is rather musical. it's hard to beat an RE20/SM7 into a 528 for a rock solid vocal sound. considering i got mine for $180, it was a steal.
i also have a Tampa, and while i wouldn't say it's "better" than the 528, it's certainly different. the preamps sound nothing alike, and the compressors are worlds apart. i like the tampa also on kick, snare, amps, vocals and bass DI.
neither is what i would call "transparent" (certainly not compared to
my minime). if i had to describe the 528 in a word, it would probably be "forward" (or "immediate") and the tampa would be "big".
if the jim williams mod makes the 528 more transparent, it could be that's something you're looking for......but it also might be at the expense of a sonic color you know, like, and find useful on occasion. i like my 528 for its color and the way it sounds as is. ymmv.
either way, considering the 528 and the tampa, i agree with Dot in that you'd be hard pressed to find something better than either in that ~$500 price range. personally i'd be saving my pennies for an API or OSA lunchbox and pres to fill it out rather than looking at $500 channel strips.
if you do get the mod, please be sure to let us know!
cheers,
wade