a couple of questions and a comment
1. Klem, when you cast the MidSpey for distance, do you hang the entire head out the rod tip?
2. What are you doing to manage your running line when you are shooting for distance?
3. How quickly do you complete your casting cycle (from beginning of the initial lift through to the final delivery)?
4. How clean is your fly line?
These 4 points are critical when it comes to effective distance casting. Probably #3 is the most important overall, as it deals with how well the rod is loaded and then unloaded during the cast. When I am working with people on their distance casting (or trying to improve my own!) I find that most of the time a caster is completing their casting cycle too quickly. Slowing down each step of the casting cycle often generates amazing results. Often in my schools I'll take a long belly line, hang the whole head out the rod tip and execute a painfully slow single spey in order to demonstrate effective and efficient loading of the rod. It would seem that with a long line out you'd need to get things moving and keep them moving quickly, but as long as line tension is maintained and a caster is properly accelerating the rod to a firm stop both during the D loop formation and the delivery cast, you can actually make the initial movements of each part of the cast very slowly. It is the combination of continuous smooth acceleration and the weight of the line that gets a really good load into the rod, and once you release that energy the line really flies.
When you are casting, keep an eye on the rod. You should notice it bent throughout the entire casting cycle except when it has straightened after the release at the end of a casting stroke (D loop formation and delivery cast). If you are doing things well you will notice that the rod is at its maximum bend a split second before you stop it. If the bend is not increasing as you are working through your cast (whether forming the D loop or making the delivery cast) this means that you aren't properly accelerating the rod. Think "slow-to-fast" thoughout the casting cycle and see if this helps to increase your distance.