I think they could definitely throw farther.
Combine the leverage gotten by the arm with the snake strike, and you have a heck of a throw. This would be on the order of magnitude difference between someone throwing a clay pigeon by hand or with a clay pigeon thrower.
What happens here is that the longer the handle, the faster (and farther) the clay pigeon will be thrown. It takes more muscle, but to have a snake strike on a being as large as a Naga, you're going to have that muscle.
The trajectory of a propelled/thrown object, which is used to calculate distance, is very much dependent on the initial velocity of the object.
The above link is the math, and the below link is the specific image on the above page that shows the correlation between initial velocity and distance, which considers air drag.
As far as accuracy is concerned, the farther the distance you are trying to hit, the harder it is to hit. That goes for everything. Getting accurate simply means more practice. Shooting a rifle accurately is pretty simple, if you're aiming for a target 50 ft away in a building. Snipers have confirmed hits at over a mile in less than perfect conditions. The thing that snipers have is training, which the military is likely to give these Naga grenadiers.
When I went through Army Basic 20 years ago, we had to learn how to throw grenades, and we had to be fairly accurate about it under standard distances. It was just a 1 day training course, but we still had to do it as an standard practice. Someone who has a "job" of "grenadiers" would have to go through training to get that position, and training continues after getting that position.
The advent of grenade launchers pretty much makes long distance hand throwing not necessary. The max range of the M203 grenade launcher I carried for my Reserve unit is 400 meters. It has a minimum safe (combat) range of 31 meters.
According to the link below, the average soldier can throw a grenade at least 20 meters, and should throw it at least 35 meters to be safe. This is because the (standard US military) grenade has a casualty radius of 15 meters. If you don't get it out there a good distance, you're going to be a casualty of your own weapon.
So, knowing that a grenade launcher can hit something at a shorter distance than a soldier can throw it, that pretty much negates the need for a "super throwing" grenadier.
Of course, that depends on your tech level. If you don't have grenade launchers, then Naga grenadiers are what you're looking for to clear rooms/trenches/gun emplacements at a distance.