I know Mosselman did the twin-turbo kits but I believe they have been bought out by Lorinser (aka CEC). Not sure if the kits are still available but you can probably contact CEC. If you are still seriously interested I may also be able to find some old Mosselman contact info for you...let me know.

John,
Skip on the oversize throttle body unless you have done some other engine work. Even with cam+exhaust+throttlebody your gains will only be top-end. (and really not that big) If I were going to do something I'd ponder a conversion to the 4-valve head, the turbo kit, a supercharger kit, or even a swap-rebuild. Granted those are all pretty big-ticket, but the gains are as well. The M103 is a top-end high-speed cruising motor. In the 300E with the mods you have it will really be a corner bender more than anything. I'd love to see your car run in a MBCA autocross/timed event.

Hope this helps...Lee