Can relate some experiences sailing the I-20 with the spin . In a number of legs in long distance racing it has been faster to sail a lower course with better VMG -velocity made good . Dependant on wind speed the crew can at times sit in and sail lower , this generally in med. wind speeds 8 to 15 max ,-a GPS will provide readings which you can verify over time with several readings over a 5 minute time frame and averaging them out . The best way however is boat to boat comparison .
Often in distance racing the easiest option is to zero in on the lead boat and duplicate their course heading .
Some of the better teams will also reverse this when ahead and try small variations of course as well as boat tuning in comparison with boats behind .-
If a boat behind is gaining on your same course this should also be what your responce is ,-trying a slightly better angle and sail adjustment settings along with more steady helming once set . Can,t tell you how many times over the years we have sailed and caught up to Brian and Jamie {team Alexanders } and had them make adsjustment with course variation and pull back ahead.
Keeping apparent wind flowing on the leeward side back is key for all sailing but particularly for the spin sail on faster cat designs .-number one is to heat it up until that flow is found then try slightly lower course angles until the ideal is found in comparison to other similar cat designs on the race course ,--It helps many catsailors to attach a set of telltails about 12 inches in from the spin leading edge about half way up ,--that is just above the seam on the I 20 spin .
When they flow your set ,--many prefer just to sail by the luff curl keeping it curing in on the edge of collapsing inward . priority is to keep the leeward spin telltail flowing .
From there it is a matter of course angle desired and how low you can sail while keeping apparent wind , this varies with spin sheeting angle and skippers ability to feel stall and flow with consistancy ,-while not oversteering which slows the boat with each rudder movement .
If you watch good teams in distance racing they are very smooth and hold a steady consistant course ,-conversely teams that finish latter will sail more eratic and abrupt course changes .
The fastest angle sailing downwind when compass course is below ideal angle requiring gibes to C mark or distance race beach finish varies with each wind speed and sea condition .

In higher windspeeds the downwind angle your able to sail and keep attached flow is much lower ,so coresponding gibe angles are much lower ,-How many have overstood a C mark in higher winds ? Apparent wind with increased speed is the reason .
Often in distance racing in higher winds and larger seas a low course is the ONLY option with the shute up .
Being on the wire off the stern with safety line holding you in place helps keep the boat "sunny side up" but there are times in larger seas it is better to throttle back to keep those bows from stuffing into waves and {arrive alive} at the next checkpoint ,-ready to sail another day !

-There is no substitute for racing time and practise ,
so get out and race when you can ,-take some lumps ,-ask questions ,-and like most all sports you will improve and eventually find yourself that lead boat .

Hope that helps
Carl