You may want to apply the carrier update just to check it. You can always restore from a backup, which shouldn't install the carrier update (at least for me).
Its not complicated at all, all you do is go into Settings > General > About and it will ask you whether or not you want to install the carrier update. Plugging in to iTunes will also ask the same thing upon plug in.
Since you mentioned the T-Mobile iPhone (assuming the AWS one and assuming you have it), there shouldn't be any speed changes since, since thats the faster one anyway. There was a hack that supposedly just speeds up the refarmed 4G (and better chooses which band to use for the AWS iPhone).