Debates are raging! Procs are being watched! Combat logs are being combed through! Cowpies are being hurled with precarious disregard! And all the while we're wondering what the final judgment will be.

Here's what we know: It's a 15% chance to proc a GCD Starfire every time that Insect Swarm ticks. There is no internal cooldown.

I will be using a slightly uber druid for stats because with 4T8, I'm assuming you've garnered some good gear. Therefore, my druid has 3k spell power, 45% crit, and 389 haste rating. As always, there is a TL:DR for those of you looking for the "express" ride.

Math Stuff

Chances to proc: Insect swarm ticks 7 times over 14 seconds. So I have
Thats 4.5 procs per minute. To even out the numbers, I'm going to be looking at 120 seconds (two minutes) so that I would get 9 procs.

Nature's Grace Uptime: I will be using Starfire's NG uptime for this so
GCD:
Average Starfire Cast Time:
Average Starfire Damage (assuming SF idol and moonfire buff to crit):
I would get 9 instant Starfires @ an average GCD of 1.065 seconds for a total of 9.585 seconds of casting time. In this time I would normally do an average of 4.5 Starfires.

TL:DR

My theoretical druid picked up 328.78 DPS through his 4T8 piece bonus assuming that the buff never overwrote itself and wasn't consumed by the casting bug. That, to me, is a significant increase in DPS and warrants the inclusion of the 4T8 bonus into the BiS gear.

A small note on all of this: The benefit of the proc decreases as Haste increases. Logically, this makes sense because there is less gain in casting time per proc as you cast faster. In Bloodlust and Heroism situations my druid is popping off Starfires at a rapid 1.434 seconds per cast. Here, the DPS boost of the proc is minimal. I speculate that Solar rotations will make heavier use of the 4T8 than Lunar rotations using high haste. For example, a druid with 700 haste will find himself gaining around a mere .9 seconds of cast time through the proc. My druid was gaining well over 1 second per proc.