Yes, it's incredibly stupid and something Ben Powell has been challenging for many years. You can see his tip "The Bigger Pool Pump Scam" on his other web site, PoolSolutions.com which he started 19 years ago. I don't know how old the article is, but it was there when I found the site in 2000. It has been a constant frustration for us over the years.
Filter prices definitely, like any decision, factor in. Here, the difference between a 150# (18") filter and a 300# (24") filter is only $150--less than ₤100 for essentially double the capacity. Plus for SwimDaddy's pool, 300# would EASILY meet his needs even with a too-big pump. And, if he can sell the old filter for $100, that's 2/3 of his cost differential between another 150# vs a 300#.
Personally, unless the filter is damaged, I'd go with down-sizing the pump and getting a 2spd. VS pumps are very expensive still, and if you do NOT have a good surge suppressor they'll get zapped. I don't know if they yet make the cut-off suppressors for them. These are sold for appliances and won't come on if you are having a brown-out (low voltage) which zaps electronics as badly as surges.
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