If I get a e14 to b22 converter, will this affect how bright the b22 bulb is? What about a gu10 to b22 converter?
Yes, they should all have enough current capacity to handle 15 watts. 

I don't need to ask your working voltage because at the lowest one likely (12V), current would only be 1.25 amps.  That's less than a turn signal bulb. 

The light will be the same brightness.  If for some reason it is less brightness, that should be considered a serious threat, as that means the rest of the energy is trying to set something on fire.  In that case search for whatever's getting warm and fix it.