I've been 5.97@116.5 in my C6 but leaving in first is HELL on the extension housing. It'll crack it then in another pass or so it pops the ears off the diff. When that happens it breaks the output shaft too. I've broken two of those housings and had to weld my diff ears twice now as well. This is with a Z06 diff so it it hits pretty hard out of the hole. It's been as low as 1.305 60ft and a couple other 1.3x 60s, but not without the above mentioned carnage.
I've tried setting 1->2 and 2->1 to 0mph and it still leaves in first. Then I sat 1->2 to 0 and 2->1 to 300mph. That seems to make it take off in 2nd when driving around but when I get up on the 2 step from a dig it seems it leaves in 1st based off the trans current gear indicator and calculated gear ration in the log. Although the trans current gear does seems to flash between 1 and 2 but the calculated gear ratio reads like it's first.
Another thing is the converter won't lock right away in 3rd, even though it's commanded to lock way before it goes into 3rd. TCC slip will show 1000+ rpms THEN TCC Line Pressure goes from 0 to 110 and it locks and slip goes to 0 right away. I'm commanding to lock 2nd now at 45mph too but even on my ET Pros I can't test that on the street. IT's worth mentioning if I have it in manual mode it seems to lock like it should.
The trans has all the Century upgrades minus the billet planetary and their TCM tune. I did the work myself. It's a YSI on pump meth and makes 23psi or so close to 7k usually.
Thanks