Learning the objective facts about Mustangs, and all guitars really, interests me and motivates my playing, so I'm glad you got this thread going. I don't have alot of technical knowledge along these lines but I'll add my 2 cents.
My pre-CBS '65 Mustang has the long-scale neck and is in really good condition. It looks almost identical to Bandit's natural finished Mustang in the photo above, but mine has the original pickups. The neck's small and fast with easy access all the way up. I prefer these traditional frets myself and don't know if RI Mustang frets are bigger. My hands are medium-sized (I'm a 6' mesomorph) and get a bit crampy after playing lots of barre chords for more than five minutes. I think a chunkier neck would prevent this but I do not know if the RI necks are the same size as vintage necks or a bit larger.
The '65 vintage Mustang's surprisingly light and comfortable to wear all night. The slab is Poplar and very thin, and these two features account for the light weight and what I'd call a "comfortable fit". After about 1969 I think, Mustang bodies became thicker and heavier, but they were given an ergonomic sculpting at the back upper-right for more comfort.
For single coils that are 44 years old, I get surprisingly little hum, like 1 on a 1 to 10 scale. Now that's at home with no neon and few fluorescent lights; playing in a honkey-tonk I might hear something different. Some folks have said their old pups are a bit thin. Maybe my decent amp, a Peavey Classic 30, compensates for that, or maybe I'm not experienced enough with more up-to-date guitars. I know that playing my bridge pickup with the tone cranked up, and with the amp gain at 1 o'clock and reverb at noon, my upper three strings give a surprisingly strong, almost aggressive lead tone and good sustain as well. This is with the amp volume on about 3, guitar volume all the way up, and using 10 gauge strings. More gain, more sustain. Don't know about feedback. I just haven't experimented with finding that, yet
The in-phase, out-of-phase switches are fun to play with. With both pickups out-of-phase and the tone cranked up, this old axe honks & barks. Then, with the amp gain between 1 and 3 o'clock, I find earthy, gritty tones...I don't get these tones from my semi-hollowbody Epi Casino. I haven't found the "heavy whipping cream" blues-rock tone which I prefer, but I do get nice grainy rock tones like Faces and early Stones.
The trem system works great...she stays in tune! And with the 10s, she bends with the best.
I replaced a broken switch; added a NOS bridge cover and trem bar, parts long lost from most vintage Mustangs. The original switch is in the OHSC, which is in very good, slightly worn condition, adding to the vintage appeal and value. Everything else is original and working perfectly. Here's the best part...
I got this handsome, great playing piece of rock histroy from a reputable vintage dealer for $850.00 The reason I got a good deal is it had been refinished. So if you're patient, willing to compromise on vintage purity, and do some good hunting, you could find a reasonably-priced vintage Mustang that's a really good player.