The Plasma Blaster is less accurate than the Riot Shotgun using Saboted Slugs. It is the same accuracy as Flechettes, but as you mention, Flechettes have 8 shots so you get a chance of hitting a little. But this is where play style will trump the raw numbers. If you are a careful swarmer, you'll be turning corners onto aliens with 2 teammates around to finish him off and you'll be happy playing the better odds of a lower-damage weapon that puts out more rounds (more chances to hit). If you're someone who often ends up facing down an enemy indoors with just 1 or 2 soldiers around, you need a 1-shot-1-kill weapon or your man is dead as soon as you press the end turn button. Not all weapons may fit your play style, but they still have their uses.
Still bringing the ball radius up to two (it's 1 now, was 3 if I recall) and/or increasing it's range or accuracy a tad might be good.
A larger splash radius is actually a problem for close-quarters combat, the role this weapon is intended for. You don't want a reaction shot to go off on an alien 2 grid squares away when the splash radius is 3.
If you're firing out to maximum range with the plasma blaster, you're probably wasting ammo.