Ok, my A1200 has Blizzard 060 and I have over 400mb Ram. MUI 5 works fine and with no noticable skowdown in either YAM or IBrowse.
MUI 5 will be required by a "soon to be released" IB2.5, which will have an updated SSL so that websites which are currently not supported can be. That's why I upgarded.
If any one wants the latest version, I recommend downloading the latest nightly build. This fixes an issue that is in the latest "stable" release which Thore is now aware of and will be fixed in the next release at the end of March.
It's best to install it in a separate directory to MUI3.8, ignore any warning that some classes can be deleted (they will still be needed by 3.

and then you can switch between the 2 just by using an assign which can be commented out in User-Startup.
I have to say that 5 does look cleaner than 3.8. Although I've had my keyfile for years and works in V5, it isn't necessary unless you want to amend and save your preferences.
All in all, I'm grateful that development is still continuing
