Well I am 80% sure that the flybacks biggest loss mechanism is the leakage...
Flyback problems at high power:
For a single transistor flyback at high power, the main feature standing in its way is the leakage inductance in the transformer, and the losses that it brings about… Each switching cycle, the leakage inductor stored energy (0.5*L*Ipk^2) is dissipated in the clamp resistor.(as you mentioned)
However, when the FET turns off, the leakage energy forces current through the primary and into the clamp, and unfortunately a significant proportion of the energy stored in the core’s mutual inductance also gets dissipated in the clamp.
(..this isn’t so bad with the Two-Switch-Flyback because the energy is returned to the input capacitor).
This latter peril isn’t suffered by eg Bridge or Forward converters, because they transfer power to the output during the FET-ON interval.
One other single-switch-flyback-high-power-problem is again instigated by the transformer leakage inductance……(its FET switching loss)…and relates to the FET drain voltage which transistions higher than V(in) due to it rising to the primary clamp voltage…..as you know the leakage inductor has to discharge somewhere and it does so by charging up the drain node stray capacitance up to higher-than-V(in) voltage and then flowing into the clamp capacitor/resistor.
…All this results in higher-than-normal FET switching losses.
So doing a Flyback at higher power would be helped by reducing the leakage inductance….eg multiple sandwich winding, & always using full-layers of turns, & preferably avoiding short bobbin spindle types.