On the MAME version, I can get around 470,000 on difficulty 4 setting. Made it to Parsec 30, but did not have a very good game. Made a way too many mistakes.
On the arcade game back in the early 90's, my friend Rich could get 1,500,000 to 1,800,000 consistently. I think we had his machine set on normal or between normal and easy. Can't quite remember though. He was very good at the game, won many local contests. I could only manage 700,000- 1,000,000 back then, he was always kicking my arse.
There is a trick you can do on Parsec 1 to get the special ship.
1. On Parsec 1, let all bugs fly to formation and do not shoot any. And do not get killed. If done properly, a shooting star will fly diagonally past.
2. On bottom row of bugs, shoot the bug 2nd from the left. A 'flag with an S' will appear and a mother bug will fly down near it. Do not pick up the flag yet.
3. Try to get your ship ran over by the mother bug. If successful, you will die and your ships will turn into the special ship. This ship seems to fire a bit more.
4. Then on your next life you can pick up the flag for an extra ship to replace the one you lost doing the trick, sweet.
Or, if you can get to Parsec 30, you can earn the special ship that way, though Parsec 30 is extremely faster than regular Parsecs.