All these things are true.
I'm not sure that you get better output vs. a ported box though. Otherwise, we would be seeing tons of PR's in comps. And the Xmax needs to be around half that of the sub from my experience.
The biggest concern is doubling the cone area. That's why I suggested two 12" PR's with my two 10" subs. I may also do both GTR's with my 12" Eclipse TI. Oddly enough though, Boston recommends the 12" GTR with the 12" G5...not sure what their reasoning is there but I trust them lol.
Btw...Boston sure doesn't cheap out on anything. They sent me an allen wrench to change the weights inside the GTR (to tune it to either 30, 35, or 45hz), but they didn't just send me a standard cheap-o allen wrench. It was a bull-head allen wrench, which generally cost more. I know, we are talking pennies here, but it's the details that count. I was happily surprised to see that they sent a bull-head allen wrench instead of the standard kind.