ul and lsa performance

[Bill Berson]

Back to the original question:
High altitude limits the performance. The solution is usually to reduce the gross weight. Floatplanes are forced to fly with reduced payload. For example, a Cessna 172 on wheels is a four seat airplane, but on floats is usually limited to two seats to get off the water.

High altitude flight conditions is a bit different than float plane issues, of course, but buying a four seater and using it as a two-seater might work. Or a two-seater flown as a one-seater... or use a loooong runway.
Bill