Not designed for this type of modificaiton.
First, it is rifle, not riffle. Second, no, or we would shoot tanks with a rifle. Cannon are more powerful. Finally, depends on the rifle. I have a .22 rifle and a .44 magnum pistol. The pistol is far more powerful than the rifle. I also have a .22 pistol, and a 30-06 rifle. The rifle is far more powerful than the pistol (in THAT example).
30-06 is much more powerful.
270
Yes. Pump, Spring and PCP (PreCharged Pneumatic) are all more powerful than a C02 system
Yes
Yes it can but getting the current rifle "Tuned" by an expert will get you better results. Tuning will make the spring and trigger perform better.
No
If you mean .17 HMR and .22 LR, yes.
Depends on the load
The .243. It's basically a necked down .308 Winchester.
Whether or not an air rifle is more powerful than a cross bow depends on the air rifle or cross bow. Generally a hunter's cross bow is more powerful than a child's bb gun. However, during the Napoleonic Wars, the Austrians had an extremely powerful repeating air rifle. It could shoot several hundred meters. Austrian solders armed with them could shoot Napoleon's solders from such a great distance that Napoleon's solders could do nothing about it. Eventually they ran out of bullets.