The '85 Bears defense was light years better than '99 Tennessee (or any other defense the Rams faced that year, including Tampa). They were also far better than the '01 Patriots defense that cheated the Rams out of their second title. In '99 NFC championship game the Rams survived a slug-fest against Tampa, but that '85 Bears defense was again on a different level - especially in the playoffs.
Bears 21 Giants 0
Bears 24 Rams 0
Bears 46 Patriots 10 (and 3 came off a first possession fumble on the 20 with the other 7 in garbage time against the second team)
In the dome, the Rams would have had a puncher's chance to replicate the Miami gameplan that beat the Bears in '85...but outdoors, in the elements (which that Rams team never had to face) I think it could have been pretty ugly game - similar to the NFC title game but without Ricky Proehl's catch...