Because of the increase in encoding speed, i've started using the -Z switch --alt-preset 128. My question is whether this will cause any unforseeable problems in regards to quality. Sorry if this has been already addressed, but I couldn't find a search string which would give me the information I was looking for...

I'd say it can both improve or worsen quality a bit depending on the case, but I don't think it will cause any bad problems.

What about use LAME 3.90.3 (-Z included by default) and use --alt-preset xxx??? Is bad or good?

-Z is only default there in standard,extreme and insane presets, not others.

"My question is whether this will cause any unforseeable problems in regards to quality."

I would guess that would have to be a no.

dammit, don't correct my verbosity!

unfortunate that -Z is unavailable for abr usage (as well as others... also liked using it with the medium preset). Ah well...