I cast these heads too and of course the ideal way to cast them is with a small amount of running line out of the guides, hence the suggestion to experiment with more to see if the rod loads up better. The ideal length was implied in other words, and the suggestion was to see if extending loads more. Yes there is a point when control is lost becase of the violation of the golden rule of fly line design at the back taper - decreasing diameter and grain weight over the progression of the length of the line. I was suggesting a length less than that, for instance 10, 15 feet kept dynamically energized to avoid 'turbulence' in the loop form. I have no problem going for 10-15 feet extra with the Rio deep sea -or- the Cortland QD as long as the line is dynamically engaged in the act of passing the shock wave thru the loop. A single false cast gets the loop accelerated for me as well, when needed. It's obviously not always needed though and any more than one is a waste.
But all that being said, if you want to cast with the proper amount (a foot or so) of running line exposed and a
crisp motion then maybe you need to increase grain weight.
The Deep Sea Rio feels heavier than the Cortland QD, I have a 325 grain if you want to experiment with it although it's going backwards. They also make a 4xx so it stands to reason that it would load more agressively too.
I am eyeing the RPLXi 10wt for a 2001 birthday/Xmas present so can't comment until I get one in my hands. I use the 9wt RPLXi and it casts a range of lines well because it has a nice sweet spot that loads up well and it can generate line speed without being stiff, which is my preference for a fly rod.
Another option would be to build your own T-style line using hi-density sinking shooting heads like the guys out in the pacific northwest (like Jim Teeny himself) have been doing for decades. SA STS tapers come in 30 foot lengths and are rated for line weight as well as density (ips). You could buy an 11wt type IV STS and chop it back a little (T-series are 24 ft I believe) and compensate for any lost taper with a 50# leader butt, tapered down to normal SWFF tippet. This would still form a nice loop. Better yet since you control the head properly cut the taper off the back end and leave the full forward taper minus the useless level front 12" section.
Anyway, good luck.