Sealed will always sound better to me if and only if you have enough output to match the output of the ported subs around tune. I am using a bass array IB in my baffle wall and my screen does not move that I can tell.
Yes it would go down. I am not sure what box sizes Ricci uses but he has shown an in room CEA of his system. The LMS 5400 is the best 18 for small enclosures as 18's are concerned. The REXXX is more like an IB driver and requires a big enclosure. I have never been technical so Ricci should chime in. You really can't lose either way. The LMS 5400 can save lots of space.