With few exceptions, I think you'll find adequate performance from most of the brand name lines on the market today, including the Ultra4.

Of course I'm probably wrong, but I can't help but think that anything changed in the design or manufacture of the current generation of lines which drastically improves tip flotation comes at the cost of durability or delicacy...due to less dense coatings at the tip or increased coating thickness at the line tip. Even if the previous statement seems completely erroneous, the following shouldn't; clean your line often, avoid stomping on it, examine it for premature wear...in essence, give it the care due to any fine piece of equipment. To help prevent the core from wicking water, you can seal the tip with pliobond, aquaseal...I hear some use super glue. Fishing strong, broken water and pocket water also make it tough to keep the tip on top. Forgive me for repeating what you may already know.

I am basically looking for a general purpose DT #5 line.
The taper on the Ultra 4 is similar to the RIO ST, but with a uniform body. If that appeals, go for it. Some other thoughts...

RIO Classic: just a bit shorter in the taper than the Ultra 4; another possibility if you like RIO lines.

SA Mastery Trout: a very compact, concave taper; good for fishing close with delicacy

Cortland 444 Peach: a supple, durable line; an affordable workhorse; excellent general purpose line.

From your post, it sounds as though you are pleased with the GLX/RIO ST match. I think any of the above lines (including the Ultra 4) would perform nicely on a GL3...and probably make for an interesting change of pace on the GLX.

And if you ever buy a floating line that sinks unreasonably despite your tending to it, consider returning it or shoot the mfr an email. Like everything else, there are the occasional duds...and mfrs realize that.