As far as assisting at race pace - a cadence of 90 or 100 is not a limit for an electric motor designed for it. With the dollars involved in cycling generally, and at elite sports specifically, I do not see any barrier from an electromechanical perspective. Most importantly for an elite rider, the motor can be optimised for a cadence between 85-95 - a very small range.
I would be less concerned about a 200W Assist over a short time, as it would be rather obvious, than a net (factoring in extra weight) 20W or 50W assist over the duration of a tour. Imagine if you could give a rider a drug that instantly gave them and extra 10% output over an entire tour? This motor could be small and hard to detect. Its heat signature would be tiny. A physical bicycle inspection would be needed.
Current battery technology limits the capacity available without obvious signs - physical size but also the weight would make the bike move differently to normal. I am sure an experienced observer would spot it. However, research into battery technology is progressing very quickly. Its foreseeable within a year or two batteries with 10x capacity for no weight or size gain will be possible. recharge times are plummeting to measurements in minute to full charge.
So should they be worried Absolutely- today I think the risk of you or me doing something and being caught is high, the hardware is too big and bulky and obvious. A simple weight test of the top riders bikes would suffice. However the dollars involved at the highest level, and the scale and length of time the cover ups and secrecy around Lance Armstrong's drug use were able to continue do not preclude a team spending a large amount of money into secret commercialisation of research results.
Within 10 years, I believe the average road or MTB will have an electric assist option as it filters up from the commuter/casual market into main stream. From there, its a tiny step into the race scene.