re: mending, it is easier to mend lighter line vs heavier and that should apply whether it's mono or braid. Having more weight that must be managed/lifted out of the water can only have a greater effect on the drift. Doubly so for those who like to hold the whole line out of the water; the heavier the line for a given length, the greater the drag that must be transmitted to the float
FWIW braid doesnt actually float (ie its density is not less than water) but it can be light enough to sit in the surface film without breaking surface tension and thus floats under the right conditions. If something makes it break the surface tension (like turbulence), then it will sink.
Has a shock mono leader been a game changer for you? Is it worth the extra visibility vs the spook factor? Keep in mind I float fish with a spinning reel.
The greatest benefit of the mono shock section (IMHO) is to have a break point weaker than the main line so if you snag up then you have somewhere that will fail and not potentially leave a huge length of braid behind. Particularly useful for spinning applications where you may not otherwise really need an actual leader.
I guess my question is more concerned with getting the best drift possible.
I guess it comes down to what factor is the most important to you in the 'best' drift, but I assume you mean trying to get drag-free as possible. From that perspective, the lightest mainline you are willing to use will perform best. Yes, there are compromises like more propensity for tangling, but that's a separate issue.
To be fair, if you're goal is the 'best drift possible', a spinning reel is not going to be the best tool for it (but you seem cognizant of that already). People can and do successfully float fish with spinning reels, but you're sacrificing a lot of finesse and ability to manage your line. Just my $0.02
Do you use your spinning rod for actual spinning applications like spoons/spinners/etc? If so, then even more reason for going as light as possible. You'll be needlessly sacrificing a lot of distance going too heavy on mainline.