Is a 13' noodle float rod any easier on the fish than a 6wt fly rod? I'm not sure... especially when one benefit is a long, long drift.
I've asked your question on forums before... and get the same types of responses. To paraphrase quite a bit: it's quite possible, but not optimal.
However, for the sake of buying a new rod, I don't think going up only one size is practical. Also, out of concern for tiring the fish, I don't think going up one size is justifiable. The proper tool would probably be an 8wt: in addition to fighting power, there's the good roll casting ability with heavy nymphing rigs and ability to use some fancier casting techniques for chunkier streamers. I've only been fly fishing 3 years or so and I have trouble casting the proper presentations on my 6wt.