Not sure if this information will help anyone, but I've done a little testing with various games at max handheld clocks with docked flags.
All games were tested at:
handheld_gpu: 460 MHz
handheld_mem: 1600 MHz
Games that run with dynamic resolution scaling:
These games will look increasingly better with higher gpu clocks. Use an AC adapter with maximum clock speeds for even better results.
Xenoblade Chronicles 2 - Night and day difference! A few slight hiccups in fps during heavy combat but otherwise highly recommended. (30fps, mostly)
DOOM - Runs at a fairly higher resolution. Looks a good deal cleaner than stock. (handheld_cpu: 1224 for smoother fps)
Wolfenstein II - Same as DOOM, though only a slightly higher resolution. Still looks rather blurry though, especially at the New Orleans episodes. (handheld_cpu: 1224 for smoother fps)
Super Mario Odyssey - Stays much closer to the native 720p most of the time compared to stock. Locked 60 fps.
Hellblade - Graphically, the most impressive of the lot. Looks much closer to 720p and seems to be bottle necked entirely by handheld gpu clocks. (30 fps, mostly)
Warframe - Another stunner on handheld. Looks crisp with increased resolution and more reflections seem to be enabled. (30 fps, mostly)
Fortnite - A good increase in resolution, though it seems to run even worse in crowded areas like tilted towers. (handheld_cpu: 1581 and stock gpu clocks for smoother fps)
Spyro - Improvements seems to be both resolution and motion blur. Looks much better! (25-30 fps)
Cities Skylines - Higher resolution, seems to also enable some kind of AA? Runs alright I guess.. fps is a bit unstable however. (20ish fps)
Yoshi's Crafted World - Cut-scene fps seem to be the only thing affected.. otherwise game play seems to be unaffected with much more time spent at around 720p.
Games that run at full docked resolution:
For the most part, these games will run at 720p, 900p and 1080p on handheld screen. Increasing gpu clocks will only increase performance as resolution is locked.
These games look gorgeous on the small screen but tend to have more performance issues compared to the games above:
Crash Bandicoot - Oddly enough, Crash 1 seems to be the most poorly performing of the trilogy. No longer a guaranteed 30fps locked, however for the most part it runs faithfully close to stock fps.
Crash 2 and 3 seem to perform better and are much closer to 30 fps most of the time. (480p > 720p)
Captain Toad - Free upgrade. Absolutely no difference in performance at all. Runs perfect! (720p > 1080p)
Final Fantasy X, X-2 - Free upgrade. Both run perfect at 1080p. Seemingly no bottle necking at all. (720p > 1080p)
Final Fantasy XII - Seemingly some fps dips at times but a much cleaner image than the stock handheld. (Not entirely sure this isn't using scaling yet..)
Crash Team Racing - This one comes down to choice. Resolutions are now at a crisp 720p, however fps takes a slight dive into the mid 20s. (480p > 720p)
Smash Bros Ultimate - Free upgrade. Same as Captain Toad runs perfect at 1080p. (720p > 1080p)
Mario + Rabbids - Fps drops at times but no slow down and is entirely playable. Looks great at 900p. (600p > 900p)
Yooka-Laylee - Low fps during the beginning intro but otherwise in game runs really well. (600p > 900p)
Games that run poorly or don't work with docked flag:
DKC - Tropical Freeze - This one is really a shame. A mere gpu overclock to 537MHz makes this game run flawlessly. Unfortunately at 460MHz there is just too much game disrupting slowdown.
Mario Kart 8 - Doesn't seem to work at all. No change between docked or handheld.
Breath of the Wild - Unfortunately this game suffers major slow down so forget about it. Either play with handheld flags or connect an AC adapter and max out clocks.
Sonic Forces - Again, game breaking slow down. Either play with handheld flags or connect an AC adapter and max out clocks or better yet, don't waste your money on this game at all..
