I cannot understand why, with a pool as small as yours, you need a 1.5hp pump, especially now that you KNOW you had a defective cartridge in the filter. Your original pump was full-rated so it's got virtually the same specs as mine.

My pool is 16x40, rectangular, and is about 20,000 gallons. I run a 2 speed 1 hp Hayward Superpump with 240 square feet of solar panels (equiv of 3 20x4 panels) with NO problem. In fact, 95% of the time I run on low speed, where it's putting out far less than the full speed 1hp.

You didn't say which model filter you had--just which line of Hayward, so it's VERY important to make sure that your pump and its output don't exceed the capacity of your filter--or your new cartridge won't last very long either.

Clearly I think your builder's solution to increase the pump was a terrible one--the original pump was probably far too big. It has two problems. First, as I laid out before, it may well damage your filter. Second it will burn a lot more electricity. A 3/4 hp pump will take a little longer to turn your water over, but will cost about 1/3 as much to do so as a 1.5hp pump.

Now I'm assuming that the new pump is the same model Hayward in the bigger size.

However, if you are happy with the results, that's fine. I simply would not recommend a similar solution to other owners--other than replacing the cartridge.

On the sister site to this one, PoolSolutions.com, there is an article called "The Bigger Pool Pump Scam!" Tip#83

This is by Ben (PoolDoc) and he wrote it a few years ago, but it is just as relevant:

The BIGGER Pool Pump Scam!
Men are sometimes really into how big their car's engine is. Or, their boat. Or, their biceps. Or, their whatever. Even some women reportedly have been caught up in the 'bigger is better' excitement.

But, as has been often observed, it's not size that counts, but how you get the job done.
This is especially true with swimming pool pumps. Flow genuinely affects how well your pool operates. But the only thing pump horsepower always affects is how large your electric bill is! What really counts is not how BIG the pump is, but how well matched it is to the rest of the pool.
A typically (not optimally) designed 18,000 gallon pool with 1.5" pipes and a 3/4 HP pump will filter about 18,000 gallons every 8 hours -- and cost about $40 per month to operate.
The SAME exact swimming pool, with a 1.5 HP oversized pool pump, sold and installed by an gung-ho dealer or salesman, will filter about 24,000 gallons in 8 hours -- and cost about $80 per month. Doubling the HP, in this case, only increases filtration by 33%. But doubling the HP does double your electric bill. Ironically, it will also worsen filtration, since the filter will be 'overdriven'! In fact, many problems with sand filters (and the sand in them) result from matching small sand filters with big pool pumps. So the next time a pool salesman tells you he'll put a BIGGER pump on if you sign now, ignore him, and ask how many gallons per minute flow he guarantees the pool to deliver -- and whether the filter is rated for that flow.