I can’t comment on the effect on the Spark specifically, but I have experience doing this to my last two race bikes. I was on an Epic, and swapped to a 120 fork; No unbalanced feeling or anything like that. I did notice a little more confidence in higher speed sections from the slacker head angle. The STA was slacker, too, but that’s easily fixed by pushing the saddle forward a small amount on the rails (I thinkI moved mine 9-10mm). The biggest gain was from the increased travel. I ran similar preload pressure, the bike just took a bigger hit before getting squirrelly. Other than that, no difference.
I’m now on a Pivot Mach 4 SL, and it came stock with 120 fork. I love it. At this point, I don’t see a need for a 100 fork on any race bike unless racing on really tight trails. If you’re comparing similar models, there is little-to-no weight gain from 100 to 120, and you can run similar preload/sag (measured in mm sag rather than percentage of full stroke), and pedal performance will be unchanged.
As others have said, dropper post will go further to make any bike feel more capable than just adding 20mm fork travel.