Thinking about new cartridge

56dB should be enough for a .5mV cartridge, but I wouldn't push it further.

I know it's hard to demo a cartridge. Could you bring your phono stage into a dealer who has a .5dB cartridge already hooked up? You could at least get a sense if your phono stage is up to the task. It would be worth the trouble because some of the carts you mention are wonderful (particularly the Lyra Helikon). Good luck.

.5 is enough for the P5. So the Lyra Helikon will work just fine. I've used that combination. Have a vkp10 now.

Sorry, I can't comment on the other cartridges for sound quality...other then it's a good list.

.5mV is a-bit bellow the bottom line for your phono sencitivity. thus you should have a preamp with input sencitivity at least 220mV or better.
if you have a unity-gain or passive preamp I would not recommend you to use lower than 0.8mV cartridge certainly depending on your power amp sencitivity as well.
the cartridges you've mentioned are all great performers but for most of them you might need a step-up transformer thus you can even use mm position in your BAT.
also you might consider upgrade of the phono as well to pass X ono used, audio synthesis... that have a max gain arround 70dB suitable to drive even helikon sl with no help of step-up transformer.