"I'm not generally a fan of such slow Mahler but when it's done with this level of scorching heart-on-sleeve intensity it doesn't really matter"
Yes, but it's applied in the wrong manner. The whole performance is laden with massive ritardandos. Wherever there's a chance to slow down, sure enough. At least Simon Rattle knows that if you're going to add a bunch of gratuitous, non-solicited ritards, then you should also add non-solicited speed-ups to counterbalance things. For me, music is not a competition about who feels the most. That sort of thinking leads to the Rollling Stones, rap, Lady Gaga, etc.
Perhaps more to the point, the performance takes a very long time, yet the ending is done in the same somewhat quick manner as in his studio recording - rather like Maazel. I'm also not a fan of the splashy, 'whangy' sounding Paiste tam-tams that they keep on using in London (while the rest of the world has moved on to Wuhans except for England and France). I also don't think that the vocals are all that great either. For me - and judging by Tennstedt's facial expressions as much as just hearing the performance - the word that comes to my mind in describing this particular Mahler 8 is 'overwrought'. Sorry, but that's how it strikes me.
If white heat intensity is the goal, then I'll stick with Bernstein/LSO and the the dvd of Bernstein/Vienna Phil. Those two are not just intense, they keep the level of concentration up from beginning to end. Tennstedt's earlier studio recording more closely resembles those two.