Trying to make a cast with a two handed rod out of a float tube is rather difficult. It is tricky enough with a floating line. It is very difficult with a sinking shooting head.
Trying to land a fish with a two handed rod out of a float tube with a 15 foot rod is extremely difficult.
One of the issues of line selection is having a head that is heavy enough to load the long rod, but it also has to be short enough for you to manage. According to the AFFTA Spey Standards Chart
on the Rio website, you will need a minimum head weight of 510 grains. A 30 foot T17 head would be minimum. Make sure to use some sort of monofilament running line for a good sink.
You would probably enjoy yourself more with a nine foot rod and a 30 foot head.