What does upgrading a Spike-O-Pult monkey's pierce do?

When you upgrade a Dart Monkey into a Spike-O-Pult, the description is:

Converts the Dart Monkey into a Spike-O-Pult, a powerful tower that hurls a large spiked ball instead of darts. Good range, but slower attack speed.Each ball can pop 18 bloons.

So the dart monkey goes from popping 1 balloon (2,0 upgrades) to 18 (3,0 udgrades).

However, what happens when you upgrade on the other path, where it says the Dart Monkey can now pop 2 or 3 balloons? Does it add to the 18? Or is it useless as one upgrade pops 3 and the other pops 18?

The 0/1 and 0/2 upgrades add to the total giving the Spike-O-Pult 21 if you have it upgraded to 3/2 just like the add to the total of the 4/0 upgrade Juggernaut changing it from 100 to 103.

The extra 3 shots many not be noticeable especially if you are planning on immediately upgrading to the Juggernaut where the base popping power is 100 so it may make sense not to those upgrades and use the extra cash elsewhere.

The catapult increases the number of bloons you can pop. The sharper shots increases the number of layers you can pop per bloon.