Was specifically thinking of switching out LFR T15 legs/hands into LFR T15 of the same slot. I'm not sure what the general % of dps increase the T15 gives you.
Most set bonuses are generally supposed to be 2-5% damage increases. 2T16 is an average bonus... 4T16 can be upwards of 10%.
Upwards of. For most people I've simmed with the set bonus on vs off, it's a 23-28k difference.