I don't think there's anything magic about a straight back 150, 172, or 182. They are all narrower, lighter, and a tiny bit faster than their back window brethren. As I recall, the straight back 182s also had the 180's adjustable horizontal stabilizer for trim adjustment rather than trim tabs--if I'm wrong, someone will correct me, I'm sure.
There's no real difference in the way that they fly. At least, I'm not a good enough pilot to notice any difference, other than the newer 182s seem more nose-heavy to me. The 150s, 152s, and 172s fly pretty much the same, regardless of year, from my observation.
The manual flaps were carried over through the E model 172s. I don't know when the 150s began to have electric flaps, but every one I've flown had electric, and I think the oldest 150 I flew was a 65 model. Likewise, the earlier window back 182s also retained manual flaps, I think through about 1964.
According to my IA, the earlier ones also used thicker aluminum for the fuselage, but again when they went to thinner metal was sometime after the back windows were incorporated into it.
All of the earlier models had spring steel gear, and the earliest stood taller, sort of milk stoolish. That might give a little more prop clearance, but not so much that it really counts. Later models of all of them have tubular steel landing gear and sit a bit lower.
There are engine conversions galore for the 150s/152s and 172s. I've flown a 152 with a 125hp conversion--makes it climb better and handle higher DAs better, but it's no faster. I've seen a few 150s with 150hp conversions, but I haven't flown any. I've flown 172s with 180hp through 210hp conversions. My own P172D has a 180hp Lycoming with a CS prop, which is a really nice mid-range conversion. Mine climbs a bit better and is a tiny bit faster than a stock 180hp 172, and of course is much better in both climb rate and speed than any of the lower hp 172s. One of the guys here has a 210hp Franklin powered 172, I think an E model, with a CS prop, and his will run past mine in all respects.
Cary