I've tested a lot of games thus far and it's been a mixed bag. Some really impressive improvements in some games while others remain mostly the same. I'm excited to do a video on this stuff.
For me, it's actually a lot of the small games that benefit the most - games which almost had stable performance but often dipped below in busy scenes.
Shantae 1/2 Genie Hero is a great one - it was mostly 60fps already on PS4 but a number of specific stages consistently ran below this (~55fps). In a side scroller, this is bad since it results in noticeable judder. Playing with boost mode 100% eliminates this - the town section on fire was loaded with slowdown but plays at a perfect 60fps now.
Mighty No 9 is an example - I know people hate this game but it was a curious one being UE3. The highway stage, for instance, suffered from near constant slowdown on a base PS4 - lots of dips and hitching. Boost mode completely eliminates those issues resulting in a completely smooth experience.
On the flip-side, I was surprised to see Shadow Complex fail to improve in any significant fashion. A lot of areas struggle to hold 60fps on PS4 which seemed crazy to me and boost mode doesn't really improve anything here.
It's a very interesting feature, though.