Yeah, I was also worried about that during Mini Jam 91: UFO since that was my first one. However, the limitations are VERY random and worth more score than the theme of the jam. For example, last one's was "Fish as a mechanic". Sure, you could replace your main character to make it a fish and cheat, but that game wouldn't be received very well on the "Use of Limitation" category of the jam, and could highly lower the score (or not, depending on the people). But yeah, the limitation could make you have to start all over again even with the theme, so it's not that easy as it sounds.
I've only ever been into one Mini Jam before (last one), and yeah; it's not very beneficial to start before the startline.
Also, as some people said before, it's almost very easy to tell if a dev is cheating starting before the jam begins:
- The game is submitted very soon after the jam begins (1-4 hours, I guess).
- The game is too worked for a jam game (Very good, fitting and smooth custom animations, music, sfx, too much levels or mechanics, etc.)
- The game could fit most of the limitations presented in voting. (On the jam page there's a page to vote for a limitation. If the game could fit more than 2 of them, it's really probably made by a cheater).
Finally. The jam is made for devs to improve or to have fun. There's no prize money unless someone else wants to give one (the host seems not to do that). So basically you're just tricking yourself into thinking you're better than everyone else. Even if you get promotion because of your game, if you continue to do this, you will eventually be caught and reputation will fall.