Thought I'd pick the collective brain on here.
I had a quick break between work meetings earlier...so I went downstairs to my basement to take a look and removed the cylinder and head off of a stock Start 5 I have. Ordered the usual parts from Treats, but just by first impressions it seems that the castings on the Start 5 are much closer to the transfers with a thin wall thickness. It's almost like they changed their cases a bit to make it more difficult for kids who bought them new to make them faster haha.
I don't know this for sure, but from just pinching to feel the wall thickness from the inside of the transfer port to the exterior under the ignition side...it seems different from my other two motors by quite a bit, doesn't seem like quite as much meat there to play with. I will take some measurements and pictures later of the cases from my flatreed, Start 3 and Start 5 just to have on here for future reference in case anyone else is interested.