What is the filter pressure?
Is the 2 HP pump a full rated or up rated version? What is the model number?
Unfortunately 2" pipe is really much too small for a spa.
I disagree in some cases. My '95 Hot Spring Grandee has a 1 hp jet pump and a new replacement 1.5 hp pump on 1.5" lines each powering about 6 jets each. That's not very many jets but I get reasonable flow and pressure out of them despite the small line size and low horsepower. My system works okay because there are so few jets and each jet pump gets 2 filters each so you don't have any constriction on the intake/suction side. Up the number of jets significantly then you'll need a much higher discharge rate from the pump to compensate so more hp and bigger diameter lines would be needed there.
In this case, the OP has a 2 hp pump on 2 inch lines and only 8 jets so I would think he should be able to get enough throughput on that system to make the jets work okay unless they really underdesigned that system.
The first thing I would do is do a quick test running the jet pump with the filter off just to see if the filter is where the restriction is. If throughput increases significantly with the filter off then either clean or replace the filter.
If the filter is not the issue then what other things could cause the system to perform poorly? Let's think this through. (Note: I am thinking here as if it's a standalone portable spa so some of the things I've listed may be N/A for your setup.)
1. Restrictions in the pump suction and/or discharge lines either due to an obstruction or the lines being undersized (personally, I don't think they are for only 8 jets but it depends a lot on how long they are)
2. Pump impeller worn out
3. Pump impeller undersized (e.g. having a 1 hp impeller on a 2 hp motor)
4. Pump motor being underpowered (design issue)
5. Pump motor being weak from age and wear
6. A leak in the suction and/or discharge lines
7. Too much pressure drop through the system due to the suction and discharge lines being too long and narrow for the pump size. (I doubt this is it given it's a small sized tub and has 2" lines in and out of the pump. My tub is much larger and therefore the lines running to the jets are much longer and I have less hp and both the inlet and outlet lines from the pump are only 1.5".)
8. Not enough current getting to the pump? (e.g. being wired for 240V but only getting 120? Is that possible? Corroded/oxidized wire connections in the GFCI box or at the pump possibly?
9. Pump motor not turning enough rpms (e.g. it's a 2 speed motor only wired to run at the lower speed or the motor is a 1730 rpm one speed motor and is supposed to be a 3450 rpm motor.)
That's about all I can think of. Out of all of the above, I would guess the most likely cause is the filter but fortunately, that's the easiest thing to test. How much do you know about the wet end and motor that's in there? Are they original to the tub? If you bought this tub second hand it's possible a previous owner replaced the motor or wet end or both with something that's incorrect like an undersized wet end or a motor with the wrong specs or got a motor that should work but wired it incorrectly so it's not putting out what it's capable of.
