The .18 will give you some more power but I'm not sure it would be cost effective.  For what it will cost for the new engine, I'm not sure you'd be getting that "bang for the buck".  Now if you had to replace your engine due to it being blown and were going to spend money on a replacement, then yes.. I would replace it with the .18.  As far as the two speed goes, there's many opinions about it.  I removed the two speed from my Tornado because I felt 2nd gear was too high.  It would scream in 1st but lag in 2nd due to the low rpm's of the 2nd gear.  Also to really get the speed out of 2nd gear you need a runway.  For me running around backyards and neighborhood streets, it runs better as a single speed.