I have experience with an SCT X-CAL 3 on a V6 automatic and on a GT manual.
Ordered the SCT with three "custom" tunes for the stock-except-a-filter-and-GT-takeoff-muffler V6, from Bamachips. It worked good. Tunes were 87 Performance, 91 Performance, and 91 Torque. All three improved the automatic shifting, all three improved throttle response, all three made the exhaust sound more "punchy".
Both the 91 tunes improved eighth-mile performance to a greater degree than the 87 tune. The Performance tune gave very slightly faster ETs than the Torque. The Torque gave very slightly higher terminal speeds than the Performance. Al three were significantly faster than stock.
I liked the 91 Torque on the street, lots of quick shove at very low RPM. The 87 Performance was nicer than stock, shift- and response-wise, but it eventually seemed to me to be wasted space on the tuner, which would hold the "canned" tunes and just three "custom" tunes.
When I transferred the SCT tuner to the pure-stock GT V8, I ordered a set of three V8 manual tunes from Bamachips, all 91 octane: Torque, Performance, Race.
I figured if I ever got into a situation where I needed to use 87 octane gasoline, I could put the stock tune back on. Stock is plenty powerful and pretty responsive on the manual V8.
Changing from V6 pretty-much-stock to V8 pure-stock involved receiving the new tunes as email attachments, uploading them to the tuner by way of supplied software. Painless, pretty much. It's also my understanding that Bamachips will update your custom tunes free, as you add features to your engine setup.
I understand the Race tune combines the low-end grunt of the Torque tune and the higher-end power of the Performance tune. The performance tune worked best at the track, principally because on the stock tires and with my stale stick/clutch skills, both the Torque and Race tunes were uncontrollable on launch.
I used the Performance tune on my 2700-mile trip through CA, AZ, NM, and TX. Averaged more than 24 miles per gallon, including some Touring at Grand speeds, some drag racing, and some mountain climbing. Rolling down the long hill from Flagstaff to Phoenix, got 31.9, by actual miles/gallons. The display showed nearly 40 for a while there.
When I got home I took off the tunes and got a free dealer oil/filter change, and haven't reinstalled the tune yet. Maybe for next Friday at Qualcomm. Getting 23.6 combined highway and around town, according to the computer, which I don't believe for a minute.
ADDED: 2014 GT 5.0A conv. black: electronics, comfort, security, CA plate (FLICK U); assembled 06-13-2014, delivered 09-25-2014; odometer today (03-21-2016) 4,321.5(--> DELETED --> 2013 5.06M coupe, black:: Odo LAST day (09-25-2014) 7,975) (--> DELETED --> 2008 GT/CS: Odo LAST day (10-22-2012): 9,719.3; #2075 of 2509 convertibles, #560 of 718 in Ebony. 2009 GT/CS: Odo last day (09-08-2012): 12,000.1; #34 of 1705 coupes, #3 of 377 in Performance White. <-- DELETED <--)