I've got no skin in the Renault/Red Bull/Lotus menage a trois. Just questioning the validity of a meaningful "real" advantage to an engine manufacturer since the regulations so heavily favor aero design.
It is indeed debatable. I mean as it is now, that the customer teams get exactly the same spec engine as the factory teams do. I guess the question is, will the factory team get engine evolutions and so on before the other teams? That was the case in the past, but i am not so sure it is as big a factor these days, because it is so competitive and the suppliers surely won't want bad publicity from the teams potentially blaming old engine evolutions for them being slow at a given circuit. We certainly haven't heard of this situation since the V8's started in 2006 which therefore tends to support that theory. There is no real reason why a huge disparity (say 15-20hp) between customer and factory engines would suddenly appear in 2014 either, when one isn't present at the moment.
Any performance gain from knowing the engine dimensions early can easily be negated. Look at Brawn, that Mercedes engine was completely bodged into that car at the last minute and it won both titles and 8 races in the following season.
The real question is (somewhat obviously) who out of Ferrari, Mercedes and Renault will have the best engine, and just as importantly, which teams will have the best KERS systems. Although even this will only come into play at certain tracks like Spa and Monza, the cars with the most downforce will generally be quickest everywhere else.