I saw a pretty big improvement in spool, EGTs, and sound by doing a straight pipe stock exhaust. I had it done for $75 and about 45 minutes. Totally worth it, and I plan on putting in a complete 4" to 5" exhaust this summer.
With your truck being stock the only gains in a full exhaust systmem would be sound. If you plan on getting a chip in the future there would be some minor performance gains and egt drop from a full exhaust system.
At this point do a muffler and cat delete unless you are getting a chip in the near future.
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