Re: Fast 6 wt or slower 7 wt?
Uplining is a great way to fine tune a rod. Does it work for every rod? Maybe. Maybe not. That's for the caster to decide. The rod weight designations are just designations. What Jerry might think is a six weight may be something that Tim or Steve might think is a seven.
Back to the original post. Will a slower seven weight work just as well or better as a fast six? In my experience, yes and maybe. It will work just as well or better in terms of casting comfort. It should feel like an old friend. It might work as well in the delivery for power, but I would be concerned about how tight of a loop that you could create with the flies that you may be using. Remember that anyone could throw a tight loop with a tuft of yarn on the end of leader. The situation is different when one ties on a fly that is heavy and wind resistant.
Back to lines. What kind of lines are you using? Floaters? Sink tips? Shooting heads? When it comes to casting streamers, grain mass of the line is everything. The heavier lines have the mass to help load rods more for those bulky, waterlogged, wind resistant flies. I rarely use a floater on my fast six weight. I mostly use sink tips with 15 to 24 foot sections. The head mass varies from 185 grains to 225 grains.