Your builder is incorrect. A 2" pipe will NEVER be able to handle the flow the pump can deliver. The pump will grind and may prematurely fail. Essentially the pump will cavitate.

I give my clients this particular illustration quite often.

Imagine trying to suck a glass of water through a cocktail straw. That straw will never give the adequate supply to allow you drink that water quickly. Get a straw from say a fast food place. They're giant and allow for sucking down that drink fast.

Pool pumps and piping are no different in principle. A large pump requires large piping to properly function.

This same principle holds true for water, air, gas and any other fluids.

Take notice at restaurants or gymnasiums where the HVAC is open in the ceiling. There will be an initial large pipe/duct work that reduces in size to key points.

Spa jet plumbing is no different. A large pipe must provide a large volume of water to the spa, and then in turn reduces down to each jet. The supply or suction side, must also be large to allow for that delivery.

We factor our gunite spa jets 10 GPM to operate properly. So we factor our pump size and then the pipe size based on the desired number of jets.

Our present build has a 10' attached spa. There are (16) jets. My desired flow rate is 160 GPM. Based on this, I know which pump I will need to deliver that GPM. Next, I calculate the pipe size. For this spa, we ran 4" suction and 4" return. All rigid Sch. 40 PVC. We use particular sweep fittings to reduce friction loss. There are other key plumbing details at the pump to reduce friction loss and allow for proper flow.

This pump, and all the other pumps on this project, are a variable speed type. By July of 2021, it will be a requirement nationwide, as per DOE/Department of Energy, to have a VS pump on new projects.

I cannot even begin to tell you how many improper set ups I have seen in 35 years. Contractors are improperly matching pump and pipe sizes. Some have even gone as far as saying the pumps are not working properly and claim a warranty issue. NOT TRUE.

About 10 years ago, a service client had a pool built with a giant waterfall that had a 5 HP commercial pump driving it. The client was told it would be like Niagra Falls. Turned out more like a garden hose running halfway. The pump grinded for a couple of months and then eventually failed. A $5 K pump. I replaced it with a 3 HP pump. The water flow was appreciably better. The problem was the piping size was too small for that pump. It failed as it never got the proper water supply and eventually burnt the motor. It overworked.

Your builder has to do some math and come up with the proper matching. I could say they are lazy, but mostly they don't know how and really don't want to learn.

Most, if not all of the pool equipment manufacturers, have a separate section in their product catalogs or on their websites to determine this matching under Fluid Dynamics/Hydraulic System/ Design/Friction Loss.

These calculations also hold true for the pool plumbing as well, not just the spas.

We use Waterway Plastics gunite jets, namely Poly Storm. In their 2020 catalog on pages 262--265, there is a chart and worksheet to determine matching. The catalog is viewable on their site.

The plumbing aspect is paramount in any pool or a spa, or a combo. If it's got the show, it MUST have the go.

I hope this information helps.