Well according to Jett engines its no issue at all, I asked the exact same question over at RC Universe and their Tech guy answered it.
From their FAQ's -

Question:
   When I put my piston at top dead center and look in the exhaust there is a gap below the skirt of the piston. I can see inside the engine. Is this a problem?

Answer:
   Almost all Jetts have this gap. It is called "sub piston induction". Many manufacturers have claimed to use this to enhance power. Unfortunately, it does not make the engine run better. However, during the design of the engine a certain amount of sub piston will allow the engine to be made more compact and thus lighter and stronger. Power is not sacrificed, but a strength and weight advantage is achieved.


The sub-piston area will only induct if the normal intake system is inefficient anyway and its most commonly found on ye old time diesels where restrictor spray bars and small crank shaft passages made it hard to get a good lungful of air under racing conditions.

Gordon, I kinda think that Cox engines with their very close fitting muffler rings scared fliers off with sub piston induction and any form of exhaust collector as the proximity of bounce back wave played havoc with its tune.

I am guessing here though that any pipe derived wave form would be very out of tune when the sub piston induction actually opens.

Cheers.