Redesigning pump and filter system

[sharpstick]

Our little(about 300 gallon 4 person?) hot tub was probably installed in the 90s before we bought this house. I'm somewhat handy and have done some repairs and modifications in my blind clueless way. (I have Google!)
1. Upgrade to larger cartridge - The old built-in cartridge filter cracked. A pool repair expert suggested just replacing it with a larger cartridge filter. He gave me an old Hayward C500 50 sq ft unit he had replaced with a larger one on a pool. I replumbed the whole mess so it was outside from under the deck so it would all be easier to work on. It seemed to work okay for several years.
2. Replacement of spa pack and 2 speed 2hp pump. A little over a year ago, the spa pack died. I finally got around to replacing it and the pump that had rusted and worn out in the meantime.
3. While shopping for replacements, I did some more random research and saw mention of a "spring loaded filter bypass valve". Hayward confirmed that my C500 has no such thing. The idea is that under normal operation, low speed pushes the water through the filter at optimal pressure. During high speed operation, the pressure is too high for the filter so a bypass opens up and llows the flow past the filter. Made sense to me, but nobody on any other forums or local pool supply shops had ever heard of such nonsense. I finally found one and have ordered it. (Waterway part number 600-8150, listed as discontinued on some sites, but I found one.) Now I plan to install it parallel with the filter. So far, this has been confirmed when I put everything back together and fired it up. The new pump is silent. The filter gauge shows about 4psi. When I hit jets to go to high speed, it went to about 22psi and started leaking out of the top gasket. I greased it and tightened it up. It seems to be holding now, but I think that the bypass is still a good thing and will let us enjoy the extra jet pressure that this pump is capable of. Next week when the valve comes, I'll take this all apart, buy more parts and reassemble it once more.
My question is, why isn't anybody aware of this? Are all new filters equipped with a bypass and spas designed with this in mind? I feel like the all knowing internet has let me down a bit this time.

 

[JasonLion]

Normally the filter is run with the full flow on high speed, or there is a separate jet pump. When using a single pump for both circulation and jets, setting up a bypass, as you describe, is a good idea but none the less almost never done.

 

[mas985]

Many heaters (and some SWGs) have a spring loaded bypass but I have never heard/seen that for a filter.

 

[sharpstick]

This brings me to my next project. I was given a larger spa for a different house. I suspect the control pack, heater and pump will all need to be replaced. It also has one of those tiny built-in filters. It is a larger spa (room for 6 - 8 people) and we hope to use it for weekend parties. Worse case, it would see more than average use for 2 or 3 days and hopefully not need extensive maintenance for the duration. (I also plan to add a separate loop through solar panels for auxiliary heating if that matters)
I now have the option to design this system any way I want. I hope not to change the plumbing on the shell itself, but plan to replumb everything to outside the box it's now in. That old box is falling apart and I plan to mount the shell in a raised deck so the rim is about 15" above deck level.) No rush on this one, but I hope to build it by this November.
These are the variables:
1. pump size - 2 hp? (or get a smaller one and swap out with the one I just bought for the smaller spa)
2. 2 speed pump or a separate circ pump?
3. Filter - I've had good luck with cartridge filter on the other spa so far. I saw one table that recommended a 2hp pump be matched with a 500 sf cart filter. Really that big? Seems like the bigger the better. The only downside is cost of the filter and replacement cartridges. Do new cart filters have bypass valves in them now? DE is looking better and better. Do they have bypass valves in them, or should I install one?
4. Order of components - Does it matter that much? Current order of the small spa is pump to heater to filter back to spa. I saw one discussion that recommended pump > filter > heater.