Hey Paul,
Yes, I'm very familiar with double figure of eights having spent a lot of time hanging from a climbing rope with just that knot and nasty death between us. For creating loops, the fig of 8 works well but for attaching bridles to spars, I use a larks head around the connector with a single loop fisherman's knot to make it adjustable. Works very well, even on slippery rope as the tension tends to jam the knots together. Still have to be careful about slippage under tension. Also, I find that 100# spectra is only 1/32"ish in diameter. I find it kind of dodgy making knots large enough that your sleeved kite lines will attach on securely with a larks head. That's I think where the sleeved bridle line has the advantage.
Thanks again for the insight!
Paul