Abraham Z,
Basic pay differences are because there is a difference between the types of job. In answer to your question, increases should be across the board.
ATCO's should, IMHO, get paid more.
As you stated, management think that there is a surplus of ATSA's- says a lot that! Better utilisation of ATSA's (and ATCO's) would even up the numbers a bit.
Not written as well as I'd have liked, but short of time.