It's about who you start with in the pens. As and Bs start together, Cs and Ds start together. Due to 'sticky draft' when you start with Cs, however hard you push you will only go as fast as other Cs (unless you can break out the sticky and latch on to the As and Bs). But when you start in an A/B pen, pushing 400 watts means I can stay with the the lead group.
As an overall race, had I started with Cs yesterday I'd probably have finished in the same position as I'd have been in the lead C group and hence would have caught the stragglers from B (i.e the guys I ended up riding with). But, I'd have done this for less effort (a lot more Cs to wheel suck and not having to go into the red early on to stick with the lead As and Bs).
So, within that race, no advantage of starting with Bs on my end result, but from a work out perspective much harder as I had to burn a lot more matches early on and then work very hard later to stick with the lead Cs.
Probably worth saying, I've never even seen the lead A/B group before when I've started in the C/D pen.
If it was a mass start, all in one pen, then zero benefits of selecting a higher cat