Since you seem to have this game almost finished, just release in (when it's finished!!). On cart, with everything on it. If it's good enough, people will buy it. Screen shots and video's will give an impression of what's different/better. Heck, we bought the improved version of Pac Man, why wouldn't there be a market for an improved Tempest Whatever (as you can see, I'm neither a game player, nor did I ever get what you're supposed to do in TE).
I am not sure how many copies of Pac-Man Tep392 actually sold. I do know he was releasing beta versions for download so people can test and report any problems. We release it to a few trusted beta testers, but won't do it publicly unless it is still in its very early stages. Right now I am in the process of getting Venture onto the market and developing a Role Playing /Adventure type game on the Atari 8-bit. Once Venture has passed all the beta tests, I will be focused on the next Tempest.
Like Tempest Xtreem, I am considering an executable version for download that is limited, see what I can fit into 64K. I figured out ways to make routines more compacted so this Tempest Executable will probably be more than just a demo.