So I've been hardcore speedrunning the SP for around 5 months now.
Up until I started really started the speedruns Ive played through the SP almost exclusively with the Blaster but nowadays as Spaz I basically only ever use the flamethrower, that bubble thing and the rocketlauncher. Blaster is only useful for breaking walls and at the points where you got no other ammo. On Lori/Jazz however the blaster upgrade is in my path so i use it after the upgrade