That's actually pretty damn good for that ram Dave... You might be able to pull like 2.5-3-3-5 (or 6)... As far as the CAS, it surely won't go below 2.5 I don't think. You could always try!
And to answer the first part of the post, you won't see a noticable difference with the minimal amount you'll be able to tighten it, if you can at all.
on a cel naw that wont help much; if u had a a64 that wld b a dff story tho cause that fing strives on timins (and 2sum xtent s does p4s but nt as much as the a64)