4.8 vs 5.3 questions

So seeing as im basically new to this forum AND trucks i had a few questions that i figured someone could explain better than google.

1. I have a 08 4.8 silverado. Who makes a supercharger for this? and why doesnt the 5.3 one fit if its the exact same setup as the 4.8?

2. Me and my buddy have the EXACT truck. Both 08 short bed regency after-style, with 275/55 20 wheels and tires. The only diff is he has a 5.3 and makes 15 mpg and i make 12 in the city. Thats with a K&N, shorty headers, and 93 octane tune from nelson. I also have a cutout and im testing that to see if it helps my mpg any. So what im asking is, why does he get better MPG than me?

3. Id like to get the acronyms cleared up so i dont sound like such a noob spelling this stuff out.
what does nnbs, sss, ect. stand for?

this forum has been a wealth of knowledge so far
i really appreciate it, thanks in advance

I'll do the easy part, nnbs = new new body style or GMT900, SSS = silverado SS. There is also an acronym guide on here somewhere. Oh for what its worth, FWIW, my 07 GMT900 got 17 mpg in mixed driving, completely stock plus the NP tune. Maybe you have a brick for a foot?

this is possible, but even if i lay off the gas it only goes up maybe 1 mpg.
bahh maybe ill change the plugs and wires? also i owned an srt-4 before my truck and the thing to get on that was the msd (sp?) wires, is it the same for silverados? and plugs, if heard its just a bunch of hype with the different stuff you can get. Any opinion on that?

factory wires are great. Stock plugs usually last 100k+ without issues. Is your convertor locking?

when you ask if its locking, do you mean is the tranny slipping? or does it try to take off with the brake on? If yes, then no to either. It does shift pretty stiff though, it did that even when it was stock.

was wondering if it locked the convertor at cruise speeds. Usually the convertor is unlocked except in OD, meaning the engine speeds change easily if you give it a little more gas while the MPH doesn't change much.

im pretty sure its unlocked, although im not really sure. Whats a good test to figure this out?
Also are you concerned if its locked and stayed locked?

I would be worried that it never locks, hard to get good fuel mileage when the engine is revving too high and not doing anything productive.

i dont think thats the problem cause it drives and rides like my friends. If you know a test i can run it through ill try it and report the results

Do you use the driver information center readout to see what your instant fuel mileage is while driving?