does the first field CAS make a lot of bandwidth difference?

2.5-3-3-8, I means the first field - 2.5.

The question is: Compared to what?

I guess it depends what you mean by a lot, but CAS latency does make more of a difference in bandwidth then any other setting with the exception of 1t to 2t command rate. In real world computing though it's not a really big deal. Your CPU speed is what really determines performance so if you have to loosen latency to clock higher it is well worth it.

Bandwidth difference?

Hmm.... In a weird sort of way I guess.

2 Modules clocked at same frequency, say 200Mhz, one with 2 and one with 2.5 will not have a large bandwidth difference at all. 2 will have a slightly better bandwidth.

HOWEVER...a module running at 2 CAS may top out at 208Mhz overclocked, while that same module might do 218Mhz at 2.5 CAS, and indirectly have a large influence on bandwidth.

Trp and Trcd (both of the 3's in your example) would have a "larger" effect on bandwidth.

Here's some reading http://www.dfi-street.com/forum/showpost.php?p=203179&postcount=4