I have two sets of wheels for my 700c Long Haul Trucker.
- Touring set has Mavic A719 rims and I keep 37mm wide touring tires on it.
- The go fast around home set of wheels is a cheap set of wheels that I got on sale, I do not recall if there is a name on the hub or not, the rims are narrower and allow a pretty narrow tire. I do not recall what model the rims are but I can say they are pretty light. I have 28mm wide Continental smooth tread supple tires. The go fast set of wheels was so cheap that I think it cost me more to put rim tape and another cassette on than the actual cost of the wheels. If the wheels cost more, I would not have bought them, I bought them on a whim.
When I put the faster set of wheels on the bike, I feel much faster. But if I go for a two or three hour ride, I get home only about 3 to 5 percent faster than if I had my slower touring wheels on the bike. And the 28mm tires (they caliper to 25mm, not the full 28 on the label) are a very rough hard ride at the higher pressure that they need. Thus, I think that the faster wheels is more perception than reality.
I live in an area that is pretty flat. While there are some hills, it is rare when I have to climb more than 30 or 40m. And I think 90 percent of the difference is in the tire, not the other items like the lighter vs heavier rims.
But if you are in a hilly area, then less weight can be much more important. But I can't imagine a lighter fork being worth it, that is a lot of cost for minimal savings - and a lot of effort to change back and forth. On the other hand, a second set of wheels can be changed quite quickly.
If you do decide to get a second fork, I assume you would keep the original fork for touring and swap them as needed. Try to get another headset race so that you can have one race installed on each fork instead of switching races each time you swap forks. I have two forks for my Nomad (one suspension, one normal) and having the second headset race is definitely worth it.
Bottom line - if you really want a faster bike, I would first look for some really supple tires, about 32mm wide or narrower. Then if that is not enough, make additional changes after that.