Having just completed a one week Regatta
in which 4 identical Sunsail Jeaneau 41s were racing
here are some observations and some things we tried to gain advantage.
All boats had identical rigs and identical asymmetrical spinnakers. The spinnakers were "small" and without looking up the actual design I can say they were high clew, medium belly (not full and not particularly flat) and probably were about 1 1/2 meters short on the luff. This allowed a decision to fly the spin high or low with the gap at the masthead or foot or balanced. The decision to fly high or low was an attempt to project
the spin above or around the main rededuce blanketing and sail deeper/faster.
Over the week we tried many things.
1/ Fly the spin low. We could carry this between 120 and 140 apparent. Shallower and we weren't getting any better speed than the jib and the leading edge kept collapsing. Deeper and the main blanked the sail. We worked luff tension and height to see if we could project
it better but really was no help. The coolest configuration we found was about 90 degrees with the jib also deployed about 60% - This ended up looking like 3 sails
and near as we could figure the jib acted as a vane to clean up the air through the slot. We only found this configuration late in the regatta
but it was really fast. We found with any more jib deployed the slot got messed up and the sails became inconsistent. We smoked everyone on a reach with this.
2/ Fly the spin high - This really didn't help much but we felt we coupld go a little deeper. Maybe 150-160. However, the boats that sailed angles travelled further but when we rejoined at marks there really was no advantage.
3/ "Kiwi" style - We flew the spin like a symetrical (guy and sheet) with no pole - no poles were allowed or provided. Only really suitable with low to moderate winds (I think we would have been out of control in anything over 10 knots) - We could sail as deep as we wanted down to like 170 which made great course but sometimes we beat the angle guys and sometimes we didn't.
4/ We also flew it wing on wing with the main one time. This didn't give any advantage over the Kiwi Style but allowed us to fly it as an asym and probabbly could have carried this in higher winds.
We mucked about with the main a bit in each configuration with more centered main working on deeper angles when flying asym but sacrificing main pressure.
Anyway all of this is jjust observable data that we collected. All in all flying as an asym and sailing angles was probably just as good as anything we did all week except thhat cool reaching 3 sail config.
The point is as the OP is discussing an asym for downwind sailing one will have to learn the sail and what works best from a VMG perspective. Most of us on the boat were symetrical spin guys with working knowledge of asym - I had flown and trimmed a giant gennaker/code zero on the benne before but wasn't/am not a symetrical spin expert. I am glad we tried all these configurations as I had personal learning
about what to expect and not expect with an asym.
The real key was sailing as deep as we could - i.e. heading directly at the finsih line would seem to be the fastest course. The angle guys would split, go 1/4-1/2 mile laterally and join us dead on at the next mark or line. They went faster on hotter angles, we went more direct.
this sailing stuff...