Latency is latency, regardless of whether a NB is involved or not (and I believe you mean memory controller, not northbridge). Latency, in its simplest form, is simply a measure of how many moves a particular memory chip needs to go from read to write. It's not exactly a speed measurement (meaning a CAS of 2 does not mean 2ms).
Look at it this way. If you OC at all, you know that the first thing you want to do is loosen up your memory timings. The reason for this (and why you'll crash an OC if you don't) is that the RAM needs to be able to take a "breather" if you're pushing the FSB high. CAS 2 reads a bit of data, then writes it to the memory chip immediately. At high speeds, this can cause a "traffic jam", as the previous bit of info might not have been flushed from the RAM circuit. Dropping to CAS 3 allows for a holding zone, if needed, so if a bit of data moves faster than the previous, it'll just cool its heels for a little bit, while the other piece moves on. Memory timings aren't absolutes. Let's say you have a gig of RAM rated at 2-2-2-6. It will run at those timings, in optimal conditions, no matter how you have your timings set manually. When you set your timings to 3-3-3-8, all you're doing is allowing the RAM to slow down a bit if it needs to
, to improve stability.
So, ironically, when you remove the memory controller from the NB, and speed it up to insane levels, the better performer will be running on looser timings.