I have high single figure hours on the Mk.26, all solo. I know a lot of real Spitfire pilots and have chatted Spitfires with them.
Within its non-aerobatic restriction, the handling and general view out, and workload of the Mk.26 seems to be very representative of the real Spitfire. Performance-wise, halve all of the performance numbers. Halve the original 4000fpm climb rate, the 340mph level speed, but also the take-off and landing distances. The one I flew was well made, would be easy to manage as an aeroplane, although in generic terms I don't have good thoughts about Jabiru engines, this one gave me no trouble and looked and sounded well: I don't know about the Subaru (?) in the 26b. Most builders have done a rubbish job of replicating the original Spitfire cockpit, and I don't know why as this is not a massively difficult task. The undercarriage was very unreliable and I was regularly needing to recycle it - I don't know if that's endemic, or just that aeroplane that week. I didn't, but it would be a very easy aeroplane both to ground loop, and to put on its nose.
If I had the money, would I own a Mk.26b? I'd seriously consider it.
I'm afraid I've not even had a chance for a good look around the outside of a Titan Mustang, so can't offer a useful opinion on that.
G