As house rules go it is one of the best balanced and "legitimate" additions to the game I have seen. Particularily if they must field 11 if they can. This would mean only if the weaker team would really require the extra attempt to score would they get it.I like the fact you've included a risk/reward ratio for the play.
However, as Shasta says it come down to a matter of controlling the clock. Slow, strong teams need 3-4 turns at least to score a running TD.
At the end of the day I think it comes down to the speedy team thinking outside the box. Make it as difficult to get the cage formed as you can, and don't always score in 2 turns. If you are facing an 8 turn cage and cannot get inside then play off the opponents team. By that I mean keep dropping back 1 square to hold him up and not get hurt yourself. He may get in anyway but this may also force him to make a last minute open field dash where you can get him.
Regardless, in your receiving half take 3 or 4 turns to score. Easy enough if you just keep running your catchers and advance with the throwers pocket. Now instead of 7 or 6 turns they only have 6 or 5. Again play a drop back game and this will force them to expose the ball to score quickly enough. Thats when you sink him.
Sorry to get into a tactics article fellas, but I think the above still makes for fun BB, without enforcing changes on the game that limits the best advantage the slow teams have. Instead of imposing restrictions on the hitter teams, I think the speed teams should change-up their tactics to control the clock themselves.