Large diameter exhaust pipes can cause reversion, which to the untrained ear will sound like backfiring. You have to find the happy medium where you get maximum velocity and minimum backpressure or restrictions.
First, 2.5 inches and up - is big for Normally Aspirated 5M or 7M. Stick with 2.25 It will keep your exhaust velocity high (a good thing), but still give better flow than the stock setup, with less backpressure (also a good thing).
Second, no backpressure is not a problem. You want as little backpressure as possible in order to get the most performance. However, most people reduce backpressure by getting bigger pipes, which like was said above, lowers the exhaust velocity and can actually hurt (robbing the low-end torque) you more than it helps for NA setups. For turbo setups like you are planning, don't worry about loss of torque because you're wanting to spool up quicker and negate that problem.
Third, (this is for another noob who asked a similar question to yours a while back) you will not backfire alot. Backfiring has nothing to do with your exhaust, headers, etc. You will get more reversion with lower backpressure, which makes a popping/gurgling sound when the cold air flows back up the exhaust pipes. This will occur even more if you run open headers.
If you are designing an exhaust system for NA setup, stick with 2.25" piping, or 2.5" MAX, and you will need a decent muffler unless you enjoy being harassed by the authorities. Add in a resonator in the middle and it will be loud enough, yet wont be a cop magnet. No backpressure is only a problem if you do so by putting bigger pipes throughout your exhaust system, because then the exhaust gases aren't able to move through your exhaust system fast enough. Yes bigger pipes will reduce backpressure, but it will rob you of lots of torque (especially in the low rpm's) in the process.
Open headers would be ideal, but the sound level is loud, and the reversion I spoke of causes damage to your valves. So while it is ideal for 0 zero backpressure, it causes other issues. Basically you need to find the point at which back pressure is minimized with the smallest amount of side effects. You want MINIMAL backpressure while MAXIMIZING exhaust velocity. That's the bottom line. Though unless you have $$$$$ to try out a million different designs to get it exactly right...just stick w/2.25" pipes for lightly modified M-series engines (I/H/E, equivalent, and less), then 2.5" for heavily modified N/A (or if you're looking to be loud) and 2.5"+ for Turbo.