Very interesting mod. I am liking the trend of more in depth thinking for the tech tree. If the issue with lasers is indeed miniaturization, it would certainly make sense to have things the way you have them in the CannonFirst or TankFirst version.
I usually research the laser rifle ASAP, so I can use it for all my soldiers until I have amassed enough plasma clips to reliably use them without much manufacturing (I am an Elerium miser). The side effect is usually that right after the rifle, I got for Alloys -> Personal Armor -> UFO stuff & Plasma, never bothering to complete the laser tree. This way would force me to get to the research that I never do.
I would, however, tweak the research times further. The laser pistol is usually a pretty quick research, which it still is in your mod. Usually, the last improvement is the hardest one to get when it comes to miniaturization (you've already used all the easy tricks). Conversely, at 420 scientist-days, the cannon takes a WHILE at the beginning, especially with a small research crew.
The total cost of the laser line in the vanilla game is: 50+100+300+460+420 = 1330 scientist-days
Heavily back loaded (rifle at 450)
In TankFirst, it is: 50+300+300+460+100 = 1210 + 420 (for the tank) = 1630 scientist-days
Roughly even but longer overall (rifle at 1110!)
In CannonFirst, it is: 50+420+300+460+100 = 1330 + 300 (for the tank) = 1630 scientist-days
Even more front loaded (rifle at 1230!)
I would consider recosting things like:
Laser Tank: 250
Laser Cannon: 170(or the other way around in the CannonFirst mod)
With the most expensive of the two being the first researchable one and unlocking the cheaper one. It's a large project at the beginning, but the benefits are obvious. Either a new weapons for crafts or a better tank to escort your rookies. Split this way, the cannon and tank add up to the original 420, but you can do the optional one later so it's no SO front heavy.
Heavy Laser: 250 (more than twice as much as the original's pistol; but once you know how to fit it on a tank/plane; it should be accessible to make it portable; also, you need to give the squaddies something to keep up with the aliens! and Heavy > Pistol)
Laser Rifle: 300 (Same as in the original, but happens after 50+250+250+300 = 850 scientist-days, instead of the original's 450. 400 scientist-days is more than a month later with the starting crew, or more likely ~10-15 days with a dedicated growing lab)
Laser Pistol: 310 (Makes it half the research time of the plasma pistol which seems reasonable)
The total for the whole line is:
50 + 250 + 250+ 300 + 310 = 1160 + 170 (for the optional tank or craft cannon) = 1330, like the original, but more evenly distributed so it's not so harsh at the beginning, and it now makes sense to research everything.
Whichever optional one is fast at 170 once you get your lab going, so it's worth it for a new craft weapon or tank.
Faster pistol than plasma to give to your explosive users is good too, especially if you use some restrictive plasma tree.
This mod would work particularly well with an alien ammo mod. It delays the distribution of laser rifles to everyone which makes the new ammo for the old rifles useful for longer.