I don't think we can say in the future with the benefit of hindsight that this engine was "de-tuned" at 650 HP, just like we can't say that the first generation TDIs were de-tuned with only 90 HP when current ones are pushing nearly double that value from the factory with only 0.1L more displacement. Technology has and will continue advance and people will find ways to wring more output out of the engines over time; that much is a safe bet.
That being said, it's the technology of the time that limits what is possible. Based on current technology, I would bet the farm that we will not see future R10s making double of today's HP numbers. For one, I said before restrictors and boost limits will set an upper limit for horsepower that cannot be breached due to the laws of physics and thermodynamics.
And in fact, if it turns out that the R10 dominates Le Mans, you can be sure that next year Diesel entrants in general or Audi specifically will be slapped with further penalties to level the playing field, such as a weight penalty or more likely even smaller air restrictors or reduced boost limits.
We will see incremental improvements as technology advances; that's exactly what Audi wants to accomplish by putting Diesel power into Le Mans. Doubling the horsepower as you propose implies several possibilities: 1) Doubling the BMEP (analogous to engine torque) and hence cylinder pressure for a fixed given RPM limit; 2) Doubling the RPM at the same BMEP as current; or 3) some compromised combination of both.
At least from the best forecast of technology within the next 5 years, I can guarantee you that we will not see 1) or 2) in the R10. Firstly, doubling PCP in an aluminium crankcase will not happen even with the best design methodologies we currently know of without an unacceptable penalty in weight and bulk. Also, the gearbox has to be considered, and significantly increasing the torque also means beefing up the transmission, which adds weight. Second, to speak nothing about combustion rate limits, a doubling of RPM in a present-state DI-Diesel is challenging because of the minuscule amount of time available for fuel injection, mixing and burning. Further, forces in reciprocating components increase exponentially with increasing RPM. Doubling mean piston speeds roughly quadruples G-forces in the reciprocating parts, compounded by necessarily heavier components for a Diesel. Assuming the engine can run unrestricted, I think 150 HP/L will be a highly challenging barrier to break while maintaining competitive weight and reliability. This implies 825 HP for the current architecture and 5.5L displacement.
Now I know the Cummins boys are going to chime in and say that the 5.9s are pushing over 1000 HP, but I'm going to preempt their argument and just ask them to compare everything that I have just talked about: weight, torque limit, RPM, reliability (over a 24-hour race with over 80% at full-throttle, not just a 1/4 mile at a time or 95% part-load operation on the street), smoke-free emissions, engine architecture, etc...