I am working part-time for a foreign start-up for 2 years, opening a beach head in the USA. They now plan to cut me short of the reaping fruits. We have no signed contract. I am now working hard to build a competing brand. Excited, yet feeling guilty

There is no need to introduce guilt into your situation, you should be contempt and justified with excitement!

well, they introduced to the market an original idea. Me doing the same thing is basically just ridding on the wave of somebody else's good idea... So that, to me, isn't very cool. that's the issue I have. And while they won't cut me off completely, they are seriously limiting my upside; thinking I should just be happy with a median salary and 'the promise' of a bonus. Screwed up from them to think I would say yes, and shut-up.

DERP !

Be a shark and get them

To be fair, you have an advantage now. They're going through the effort of establishing the market in the first place, which is expensive and time consuming - don't forget that just because you're first doesn't mean you're best.

You're uniquely positioned to use their market development to leap-frog past them with a better/faster/cheaper/more awesome version of whatever it is, precisely because you KNOW them intimately - strengths, weaknesses, the development roadmap, etc. Go get those bastards.

But next time, protect your shit first.

Go get them! Anyone who goes back on their promise deserves the repercussions!