Hello MC Studio,
2xCMYK should be around 40% faster as you can achieve the same result while using a lower printing resolution. Which is the software you are using and what resolutions did you use during your testing?
I use this setting all the time and the printer is much faster, , much less maintenance and longer lasting print heads as no white ink to cause problems, colours are the same as the 1c setting the only issues is that because twice as much ink is going down if you have thin garments the ink may bleed before it gets heat treated.
1 issue with texjets printing onto light garments with no pretreat is that the black always goes charcoal/dark grey on heat press so needs a double pass to stay black ish, this means some colours will darken due to volume of colour ink on second pass if you do not adjust the original artwork. Would be great if there was a setting to wind the colours down by another 50% but keep black on 50% default so that on light garment double pass printing it would not be necessary to adjust the artwork. I have this problem on Dupont and "advanced" inks, some garments worse than others but the black always goes dark grey on heat treating esp on default 50% single pass