A few more runs tonight and a bit of experimentation. Score:
4,311,370 - Spyke(Ant) - 5-5 - PS2
Found that when you power up beyond max, you get a P under your remaining ships in top left corner. These are cumulative and adding further "P" icons will add to the store under your ships. I THINK it gives your new ship a boost when you die, but it increases rank too, making you more likely to die in the first place...
Also, found that exceeding 5 bombs in your store increases rank. When I kept it to 4 bombs, the bullets were less fast.
It really does seem to be about "managing" how the game responds to your play. Shame as there are so many easy point bonuses lavished on you in the game, but that path leads to a fast death. At least for me anyway!
I find level 4 pretty hard no matter what precautions I've taken but at least I can get through the boss with no losses now

Finally, I think the purple P pickup is worth the increase in rank regardless. It acts as a one-hit-shield. Ok, so you get hit and the shield goes but you don't have to stop and start again - you can maintain rhythm and stay in the zone.
Brilliant game!
Ant
Jp-modded PS1 - PAL PS2 - Jp PS2 - Jp Saturn - PAL Xbox 360 - Jp Xbox 360 - Jp MD - Multi-region DC - Spare DC - Pal GC - GBA - DS-Lite