I am interested in the question of whether
flat top pistons are superior ... either for
most effective quench,
or faster flame speed,
or fewest hot spots,
or conducting the least heat toward the ring lands,
etc

This article about the GM LS7 engine

http://www.popularhotrodding.com/tech/0504phr_chevy_ls7_engine_review/index.html

has the quote

Special cast pistons were developed for the LS7, which feature an anti-friction skirt coating, anodized ring lands, improving hardness and wear resistance. Larger-bore pistons are by nature heavier than their smaller-bore counterparts, and here the LS7 design team countered this tendency by moving the pin bosses inward and employing a shorter high-strength full-floating piston pin. This approach yields a piston assembly that is lighter, while the shorter pin represents a mechanically stiffer arrangement. With a flat-top design, the pistons deliver a compression ratio of 11:1.

the picture has the caption:

Topping the rod is a flat-top piston helping the LS7 achieve a compression ratio of 11:1. The piston features a skirt coating to reduce bore friction, and hard-anodized ring lands, which enhance durability and temperature resistance. Note the short compression height and the lightweight reactive ring pack; pins are full-floating.