You can convert the mid-motor car to a geared diff as shown here:
http://www.oople.com/forums/showthread.php?t=143535
It isn't cheap and the three blokes who have purchased the first batch have grabbed a bargain.
I'm going to make another three and one will go on ebay in the next couple of weeks at a starting price that will include world wide shipping.
Or you could just PM me to order one of the batch. It is necessary to modify the T-Plate and the Rear Brace but if the customer sends me their parts I will modify them and reduce the price accordingly.