Lots of variables in your question. With a full cat delete and Magnaflow you can expect maybe 35 - 40 rwhp gain. With just the muffler and leaving the cats, not much can be expected, other than better sound.
The K&N filter drop in is the way to go for hp. The CAI system has no effect as tested by DC Performance and X-Metal. (In some instances it dropped the rwhp.)
Now about the mileage question...if you are worried about milage, you have the wrong truck. It should not be an issue, much the same about the cost of using Mobil 1 Syn, if you cringe at the cost of a quart of this stuff...you have the wrong truck.
Just my opinion...others will chime in...
Welcome! You will find this a wonderful place to spend a few hours...every day!