Assuming your GCD was 1.12 seconds and the PtL proc would GCD-cap you, by refreshing the LB first, you'd get:
- 1.12 secs more of LB uptime, at the cost of
- pushing your rotation back by 0.12 seconds
I'll take the numbers from my rawr set-up for this example: I'm set up to do 20031 dps, and one LB use is worth 25290.972 damage, over its 12 seconds (very roughly speaking; we know that LB damage is actually much more back-loaded than evenly distributed).
1.12 sec more of LB uptime would therefore translate into about 1.12/12 = 2360.5 more damage. However, being set back by 0.12 seconds would cost 0.12 x 20031 = 2403 damage.
So yeah, it's pretty much a wash if your haste is such that you have a 1.12 sec or so GCD, which is I think anything less than 900 haste. Above that, increasing your LB's uptime would become the favorable option because you're not setting your rotation back that much.