the tuning will help the mileage more than an intake or an exhaust. but combining the 3 will net you the best mileage. banks ram air (if they make it for the 2013s), turbo back (ditch all that emissions junk) and efi live.
on my 05 i've picked up ~3 mpg with a custom cold air and a downpipe back straight pipe and a ppe tune (will change to efi live soon).
UWUA AFL-CIO Local 1-2
'liLLY' 05 duramax 2500 CCSB 4X4- 334,995
Limitless Diesel Built trans with Suncoast 1054 converter
, Grade C's, ARP head studs, PPE Standard, Edge insight cts w/ EGT probe, Duramax Tuner race valve (plug), Magnaflow 3" downpipe, 5" flow pro, 265 transforce, Bilstein 4600, pcv rerouted, egr blocked, unplugged & bypassed, kitty delete, AFE turbo inlet, Reese wd/ anti sway hitch (cam style with trunion bars), Pacbrake airbags, 110 gallon aux fuel tank, Merchant 'all on' module, transfer case rebuilt (merchant delux bearing kit, pump rub kit and case half), home made winter front, duraflap front and rear, new rockers with bed liner