Certainly in the UK, performance write-downs are not uncommon when an engine (or more to the point, powerplant:airframe combination) is out of tolerance but no fault can be found which is likely to degrade safety (beyond the loss of performance), and the performance still remains within certification requirements.
Given that in the UK it's generally CAA Flight Test department at Gatwick who issue these, you might be best contacting DGAC's flight test department for guidance.
Having said that, if you've got (presumably) a single rotor multi-engine helicopter, then with two different engine power output values (I assume that you're not de-rating the other engine to match) you may well be putting unacceptable internal loads or vibration characteristics onto the gearbox. In your position I'd want a clear written assurance (usually called a "Letter of no technical objection") from the TC holder that this was acceptable before accepting a performance write-down based upon a degraded single engine.
G