Removing the muffler will not effect the precious back pressure.....lol.I don't know if a tune will fix this or not, but I do know you'll lose power without an exhaust. I cant give you the specific of why, but your engine is build to run best with the back pressure that is created through your exhaust. I know because I ripped my flex pipe in two once and the car was a dog until I got it fixed.
Um yeah? Back pressure in the exhaust system is created through the converter, muffler, bends in the pipe, and any other obstructions. remove any of these obstructions and your changing your back pressure and the performance of your engine. Now I'm just passing on information on read/heard but guys who have done this can gain horse power but lose in torque which will make the car feel groggy at lower RPMs. You wont know how much of an effect the lose of a muffle in the system will have till you try it but there would be an effect.Removing the muffler will not effect the precious back pressure.....lol.
Should google exhaust back pressure and find out what it is and how it works!
OK. I'll give you the specific terminology and technology behind some of this stuff but the simple fact remains that the OEM tune was done using an OEM exhaust so changes to that will effect performance (maybe not alot consider the small change that is made) unless some retuning to counter. I've never heard of a Top Fuel Dragcar or an F1 car that hasn't been properly tuned for every change that is made to it's engine so that argument is null.The term back pressure is always misused in the context of exhaust!
Back pressure is nether wanted nor required to help an engine perform. It is exactly the opposite. Have you ever seen a 6500hp Top Fuel Dragster with mufflers? According to your theory these engines would have more power if they had long exhaust systems with back pressure in them. But they don't! Same goes for F1 engines.
When you crack your exhaust or put a hole up close to the engine it's not the reduction of back pressure that causes the change in performance. It is the change in how the pressure waves act inside the system. The power is still there, it will usually be in a different area of the RPM range. Usually up higher!
Here is a spot on explanation of back pressure and exhaust scavenging, I didn't want to have to go into my fluid dynamic books and regurgitate, so I Googled it!; http://www.dsmtuners.com/threads/exhaust-the-straight-scoop-on-backpressure.168578/