Hi Scott,
A 5000' density altitude will obviously decrease performance. I do not have experience with ground operations in a 150 at those density altitudes. I have cruised it at altitudes as high as 12000', but it sure takes a while to get there! My usual operations are from less than 1000' ASL runways, though on days as hot as 100F. Performance suffers, the flight manual describes it well. The climb prop is the way to go, don't bother with cruise props on C150's, the plane just does not have the good aerodynamics to operate well at the higher speeds, very shiny C177's are good Cessnas for that.
The type of operation you plan would certainly benefit from a four cylinder EGT (I have an 8 chanel EI instrument, and highly recomend it), this would allow you to lean during takeoff.
As for the 150HP 150, I'm not a fan. They can have poor useful loads, and mushy handling in pitch as a result of weight in the tail. They do not benefit from the added power at low speed. They are generally very expensive for what you are getting. For the cost, you may as well buy a 172, and get the extra room inside. Cessna has an excellent product range, so you can select a model with just the characteristics you need. Aside from STOL kits on the earlier wings, I don't see much value in spending a lot of money to try to convert one model of Cessna to be more like another - just buy the one which best suits the role.
As for the glide with a STOL kit, gliding more slowly than the factory glide speed has no benefit, if safety and distance are the objective. If you glide more slowly with the STOL wing, with a plan to increase the speed near the end to enable a flare, you'll give up more altitude as you lower the nose than you would have saved flying more slowly (and it's the worst phase of flight to be fooling around with an unstable approach). It's sort of like gliding with 20 flap, and then retracting the flaps in the latter phase of the glide for more distance - you'll loose a whole bunch of altitude as the flaps come up, and any drag reduction advantage is lost right away.
Flying C150's overgross is a poor idea, they just do not have the power to carry it off safely, and there's a good chance that you had to load an aft C of G to get to that weight - unsafe. The 150 is an economical way to fly, so expecting more than what the performance figures offer is just not realistic. The C182 has remarkable performance and capacity, and very early ones can sometimes be had for not much more than twice the cost of a good C150, but the operating cost are a lot more. I cannot speak to higher weights with 150Hp C150's, the one I used to fly was still a 1600 pound gross weight. The C152 is 1670 pounds if I recall correctly, I don;t remember how much of this is extra useful load.
As for flaps, aside from occasional zero flap pracitce in case of system failure, I fly all takeoffs at 10, and landings at 40. My reasoning is simple, flaps cost you nothing to use (I've never seen a worn out Cessna flap motor!), brakes, tires and shimmy dampers do. If ground speeds are lower, costs and risks of damage are lower too. A lot of my operations are from less than perfect runways, so I try to minimize wear and tear on the airframe, I think lower speeds on the ground helps with this.
That's all for now, Pilot DAR