If you get rain remember the lid sits INSIDE the center section so, you are pretty likely to get water intrusion from the rain, find some way to get some cover,
Also, you are not doing 8# roasts you are doing 16# of probably fairly cold meat. If you are not using the water pan(except foiled to catch grease) your 275 should not be a problem but, I think you’re going to need more time.
The barbecue gods do not like to be rushed, it might be 10-12 hours or possibly more. If you get them done in nine that’s pretty good. I tend to plan on an hour a pound with an extra hour or two for cooking if they get done a bit early, you can always foil, wrap in towels and let them rest in a cooler for a couple of hours before pulling.
My last butt was huge, almost 12# and took 17 hours. Just something to keep in mind, that’s the biggest butt I’ve ever done.
You will likely need to replenish some charcoal, I just use a fireplace shovel through the door, easy enough.
Good luck, let us know how things turn out.
Butts are pretty forgiving but, the stall will stress you out the first time, just be patient, you will be rewarded.