Thanks cmonkey, and I wouldn't be able to hit those scores without your help - knowing how and where to search for the S-pacs, knowing the rules to always find the hidden power ups and getting the 6x7650 balloons (which I hit most of the time on trip 1, at least) are all very important parts of my gameplay, the S-pacs in particular are worth a big chunk of the scores.
On MAME I have got over 1.6M, and I think if I could play a "perfect" game (no silly loss of life, no deaths on "easy" rounds, and a bit of luck on some scoring opportunities) I think I would max out at around 1.9M. To beat masa.jp's score, I'm going to need to work on a handful of "hard" rounds - I have a shortlist of around 6 rounds - and develop some better strategies for those in particular.
At present I don't think I'm ever likely to get close to the 4.1M world record. That just seems a very very long way off!