Wilfred - I'm not sure what your requirements for dynamics and punch are, but in my experience the 845 tubes can deliver plenty of it - I'm guessing this would especially be the case with Jim Smith's Viva mono blocks, which appear to be sporting a small regiment of 845s there.
Aside from that, to my ears the 845 is a far superior tube to the KT88. While the latter are fast with extended highs and tight bass, they lack that SET magic that the 845s do have a taste of, and which I would certainly be seeking to get with a pair of Westminsters.
In that regard, I also find Jeff Day's comments about the synergy with the 300Bs quite intriguing. Indeed, I'm guessing that this is more a question of synergy than outright power. If I were you, I'd be seeking out the former rather than the latter. I'd also certainly be pinging Jeff Day for his thoughts - I have no doubt he'd be more than happy to disgorge his wisdom for you.