I wouldn't really say it is a boost per say. It is definitely a volume boost. I find that it works great as a volume boost with my Fulltone OCD and channel 3 of my Roadster -- great lead tones and plenty of sustain. The SOLO button really helps kick the volume up when necessary.
I used to try to use it to boost my solos. It works fine, but still, I don't think there is really a great lead tone in the Roadster as is. I used to, then I tried the Fulltone OCD. Now, I can get some gritty lead tones, with tons of sustain, similar to the Mark series amps. Using my Fulltone, I now don't have to crank the gain on channel 4 just to get lead tones to my liking. I now have four channels, and with a stomp, I can get some great leads.
Try the OCD!