What percentage of pressure loss is there between the cylinder & forcing cone.
For example,if the same cartridge was fired in a closed chamber like a rifle,what would change,and by how much?

For all intents and purposes, in an FA, it's negligible. My FA has only .001 gap and that's been OK since I've had it.

I don't know what the pressure loss is, but can gander a gess what the velocity increase will be. First you have to add about 2" to the barrel length of a revolver to relate it to way barrel length is measured in a closed breach gun. IE, a 7 1/2" revolver would equate to a 9 1/2" closed breach.

In testing I've done comparing 7 1/2" revolvers to a 10" t/c contender, I've found the contender picks up a bit over 100 fps over the revolver, even though the barrel is less than an inch longer, and straightwall cases gain about 30 fps per inch of barrel. So, I'd gander for the same barrel length, no cylinder gap will gain you 100 fps.

I'd venture to say that peak pressure is the same for the revolver as the gapless gun, but the pressure/time curve is different as the pressure will drop faster in the revolver, and hence the loss of velocity as the powder can't do as much work excellerating the bullet when the pressure drops off sooner.