Depends I suppose, as the weight of 27s varies because of individual detail differences, though it’s not massive- I seem to remember that the weight of a 33 (dual braked and ETH fitted, as all were in the 70s) is about 77/78 tons.
The usual vacuum-only, boiler-fitted 27/0 which hauled the West Highland trains weighed in at about 73 tons. A dual-braked 27/0 (034, 038, 041, 042) or 27/1 with a boiler probably weighed about 75, and a dual-braked ETH-fitted 27/2 about 76. A vacuum-only non-boilered 27/0 in the 27024-27031 series probably weighed only about 71. I believe a 27/2 is at the high end of the RA5 bracket, with a 33, which is a couple of tons heavier, just edging into the RA6.
@CheshireScot I don’t know if this was anything more than idle gossip, but after the plan for 33s was shelved, there was some deliberation about 31s (presumably these would be boiler-fitted 31/1s, as 31/4s are RA6 so no good), which would presumably get sent north from the Eastern region instead of the 37s. I can’t help thinking that this suggestion was hopeless, as a 31 would hardly perform better than a 27 (despite being as heavy as a 37 and probably using as much fuel as a 37). I don’t think a 31 has any more tractive effort than a 27, if my memory for these things is correct, though they’re probably less prone to slipping. The ScR had no familiarity with the class in any case, and I just can’t see what advantage they would have brought. Perhaps they caught fire less often than 27s? Clearly the obvious solution was the more reliable and powerful, but still RA5 37 (with which ScR drivers and depots already had some familiarity) which dominated the line until 2000.