Hey BlueCrew,
I fish senkos using braid as well. Really, I don't think the use of braid makes a huge difference. I prefer wacky-rigging my senkos, which means I tend to fish them in lighter cover, so I like the lighter test lines. I also use a FC leader because I feel that with the slow drop of the senko, the fish get a better look at the lure.
The city ponds that I fish in are never really deep (10-15 max in the center) so the theoretical slowing of the lure has never impacted me, although I suppose in deeper water it could. Also, with 20# being so thin, I don't think it offers significant resistance to slow the lure's decent noticeably, although I think that higher test (e.g., 30, 40, 50, etc) could start to add up. The real beauty in fishing is that sometimes that's the ticket, and so using braid because of the effects could be the key to catching fish (e.g., darker lighter conditions, colder water, night fishing, etc.).
If I were you, I would decide on the line test by the conditions (i.e., the level of cover). I have pitched senkos (t-rigged) into heavy cover on 65# braid with success.
Does braid effect the way soft plastics fall? In theory, yes, it slows it down. Noticeably? Probably not, especially if weights and short depths are involved. However, I could easily imagine a weightless senko thrown on 80# spiderwire to fall noticeably slower than on 10# FL.
Hope this helps.