i got a 49 prepped, i havent geared it yet, but i cant see it being better than my 50.
the HP Pool seems to be pretty much the same on Npcs in TBC anyways, roughly 11k but my 49 ungeared tank, is about 11k hp vs my 33k hp on my lv50
i have all the azerites/essences on my 50, and yet to be determined, i have access to Shadowlands gear that is ilvl158 on my lv50, that i dont think a 49 will be able to come close to.
i read somewhere, that someone got into SL on a lv49 and it got ilvl88 drops from quest gear, which will beef then up a bit, but if the Health pools for mobs at lv49-50 stay the same, i reckon a azerite equipped lv50 with ilvl158 on everything else, will be OP at lv50 farming old content.
plus is you are looking at boosting, you have the EXP Eliminator buff, it will stop you on your lv49, but if you play a BFA account as well, you can transfer your twink from SL account once geared back to the BFA i believe and rock the unlocked Bgs/Dungeon content