Spent the weekend sealing my attic cans and learned a lot

I did somthing similar last winter with the older Halo cans in my hallway that defintely weren't IC-rated. I was worried about the heat buildup too, but I ended up going the DIY route instead of the pre-made covers. I used scraps of 5/8-inch drywall to build little five-sided boxes for each fixture. It was a slow process measuring and screwing them togather in that tight crawlspace, but it saved me a decent chunk of change compared to buying the Tenmat covers at the big box store. I used Great Stuff Fireblock to seal the base of the boxes to the attic floor. One thing I noticed was that even with the boxes in place, I had to be careful not to pack the loose-fill cellulose too tight against the sides. I kept about a 3-inch buffer around the perimeter before mounding the insulation back over. Since finishing the project, those cold spots in the hallway are gone, and I don't see the 'ghosting' dust patterns on the ceiling anymore. If you still feel a tiny bit of air, check the trim. I swapped my old ones for airtight baffle trims with the foam gaskets. It realy helped stop that slight whistling sound during windy nights. It’s a messy job, but it makes a massive difference in how the house holds its temperture.

I acutally went a completly diffrent route when I dealt with the drafty 6-inch cans in my 1970s ranch. Instead of trying to seal the old housings, I pulled the entire metal cans out from the ceiling and replaced them with those ultra-thin LED wafers, specifically the ones from Halo that have the remote junction box. Since the wafers are IC-rated and only about half an inch thick, I didn't have to worry about clearance or building boxes at all. The best part was being able to lay the R-38 fiberglass batts directly over the hole without any air gaps. I used a bead of silicone caulk around the trim of the new light to ensure a 100% airtight seal against the drywall. It took about twenty minutes per light to swap them out, and I didn't have to spend hours in the attic fighting with spray foam or measuring drywall scraps. It cleared up so much space up there and I stopped seeing those little dust patterns around the lights where air was filtering trough.

I'm always a bit wary of sealing those covers up too tight with foam. Have you ruled out the possibility of moisture condensation building up inside the enclosure? If you've got any humidity in the house, trapping it against a cold attic surface like that can lead to some nasty surprises with the wiring or the drywall later on. Also, are you sure the draft was just the can itself? I've found that alot of times people seal the fixture but ignore the wire penetrations thru the top plates right next to it, wich can be just as bad for heat loss.

While your application of fire-rated covers effectively mitigates the convective heat loss associated with the stack effect, one must also account for the conductive heat transfer occurring thru the metal housings themselves, as these essentially act as thermal bridges that bypass your attic's thermal envelope. I’ve found that even with a cover, if you haven't meticulously verified the seal between the plaster frame and the gypsum board—perhaps using a high-temperture silicone rather than standard expanding foam—you might still be subject to minor bypasses. Did you happen to measure the temperature differential at the ceiling surface with an infrared thermometer after the installation to verify the uniformity of the thermal barrier?