I'm told ground is proficiency based but that in the plane you have to fly up to 25,000 feet! I don't know anything short of a turboprop capable of doing that, let alone have the ability to rent it. I'm finding different answers everywhere I look! Can anyone cite the FAR on this?
You seem to be confusing a high-performance endorsement with a high-altitude one. Both are in 14 CFR 61.31. They're too long to quote here in full, but the high-performance one comes down to this:
(i) Received and logged ground and flight training from an authorized instructor in a high-performance airplane, or in a flight simulator or flight training device that is representative of a high-performance airplane, and has been found proficient in the operation and systems of the airplane; and
The high-altitude one is where your 25,000' number comes from:
(g) Additional training required for operating pressurized aircraft capable of operating at high altitudes. (1) Except as provided in paragraph (g)(3) of this section, no person may act as pilot in command of a pressurized aircraft (an aircraft that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet MSL), unless that person has received and logged ground training from an authorized instructor and obtained an endorsement in the person's logbook or training record from an authorized instructor who certifies the person has satisfactorily accomplished the ground training. The ground training must include at least the following subjects: [...]
As well as the ground training, you have to do (at least) the following in an aircraft or approved simulator:
(i) Normal cruise flight operations while operating above 25,000 feet MSL;
(ii) Proper emergency procedures for simulated rapid decompression without actually depressurizing the aircraft; and
(iii) Emergency descent procedures.
There is not a whole lot of difference between flying a 160 hp Skyhawk or Cherokee and a 200+ hp “high performance” Skylane or Cherokee 6. About the only thing I can think of is that you need more right rudder on takeoff. They are usually heavier airplanes so ground control is a little different as well.
I suspect the high-performance endorsement is a holdover from the days when students learned to fly in 65 hp Aeronca Champions since the definition of high-performance from 61.31 is:
f) Additional training required for operating high-performance airplanes. (1) Except as provided in paragraph (f)(2) of this section, no person may act as pilot in command of a high-performance airplane (an airplane with an engine of more than 200 horsepower), unless the person has—
Most airplanes with more than 200 hp also have a prop control as well as a throttle. So for most people, the biggest thing you need to learn is how to manage the manifold pressure (MP) on the engine with the throttle and the prop speed (RPM) with the prop control.
If you haven’t used a constant-speed prop before, it might take an hour of ground instruction and an hour of flying. If you already know how to use a constant speed prop and how to read the flight manual pages regarding various MP and RPM settings then it might take 10 minutes on the ground and 3 takeoffs and landings.