I've tried fire line on spinning reels. Very stiff and I hated how flat it was. I ran yozuri hybrid on one of my first baitcasters and liked it, many many years ago. If I remember correctly it's somewhat stiff. To be honest the 20lb braid is the 6lb diameter, or 15 is 4lb, but I wanted to get it because it's just as thin and strong, and braid doesn't wear or or keep memory. In my experience on smaller reels, memory becomes an issue wether the line is "low" memory or not. That is why braid appeals to me. I was looking at suffix mono in 4lb as well though, it's cheaper too. I could go with 8lb braid which is 1lb mono equivalent, but going that thin would make me worry about snapping the line on longer casts. I may go to a 5'6" rod, but I don't think I'd go higher. The spots I fish spinning gear are tight. My 6'+ rods always grab a branch no matter how I cast. But these spots produce nice trout so sometimes I step into the water if it's a warmer day. I'm going to use my current UL setup today. An Abu cardinal 101 on a shakespeare excursion 5' UL 2 piece and some 4lb stren mono. I got it for free and it's what got me into the UL idea. Landed some decent trout on it last year.