When we looked at the v4 model we decided that there needed to be a huge change in how we store text and tokens to improve performance for large files. We have been building the WPF version of SyntaxEditor for a while now, and that is where we've been prototyping out all our ideas for improving nearly every feature area of SyntaxEditor, large file performance included. For instance I can open a 10MB C# file and start typing instantly with that newer design.
Our plan is to get the WPF version to support the same feature areas that are in the WinForms version and then hopefully migrate the WinForms version to use the newer design so that languages, etc. can be shared between WPF, Silverlight, and WinForms SyntaxEditor. We continue to diligently move forward so that we can get to that point, but there are still a number of feature areas left to implement in the newer WPF version. We are working on it every day though as it is a top priority for us.
So again to answer your question, yes we have made very drastic large file handling performance improvements. But they are currently in the newer platform we are prototyping out with the WPF version, and we hope to eventually get them into the WinForms version as a 5.0 version.