you will get a few answers that differ on this subject but my experience shows that if you had two models the same sat on the runway, one with a bypass and one without, you will find generally the one with a bypass has a faster top end speed and also carries the energy better in the vertical, out the bottom of a loop and so on. A lot also depends on the airframe being used. A good argument for a model that would make good use of a bypass could be one with big inlets and lots of internal formers acting as air brakes. Some also say that a bypass forces high speed cold air into a hot tailpipe which makes it expand and gives better high speed thrust. Others will say that you will get a better static thrust without a bypass. Maybe this is true because the engine could get a better supply of air through gear doors etc and not sucking up though possibly long inlets without the ram air effect you get in the air.

One experience that springs to mind was about 12 years ago when me and a friend both built BVM Mavericks. My friend had a very good 12.75lb Wren 54 and mine was a 12lb Wren (I built and tested both engines so I know the figures were correct) My maverick had a bypass and my friends didn't. Although he had 3/4lb more thrust than me, my Maverick was way way faster. Also this was in the days of regular flame outs and mine was seen to retain it's speed much better.

I don't think anyone on here can honestly give you proper calculations on what the difference will between both installs as there are so may variables in so may airframes, although some may try and baffle you with bullpoo pretending they can..

Jason