I think you meant something else? You said,

• Asmallworld lets you signup for CX SL for USD 110 if you're US resident
• Founderscard lets you signup for AA PLT challenge for USD 395.

AA PLT = CX Gold not CX SL so I think you meant to ask whether MPC SL worth USD 110 (consistent with the title of this thread)?

Everyone has difference experience but in general I think the answer is NO.

• By nature someone who is looking into this is someone who doesn't fly a lot so the status is only useful once or twice a year at most.
• Others above mentioned that it gives you access to advance seat assignment, CX lounge, etc etc. True but as you mentioned, in order to sign up you have to be a US resident. So how often is US resident (i.e. based in US) going to fly CX? There's always exception of course (very few things are absolute in life) but if someone who is a US resident flies CX quite a bit then the person should, in theory, be able to get to CX SL through flying, no?
• For US domestic (since the person applying is US resident so presumably fly a bit of US domestic flight as well) it doesn't mean much. No AA lounge access and can only access MCE at check-in (which usually means middle seat ...)
• It does give you one free checked bag for your occasional US domestic flight but it is not really useful considering 1) the elephant-size bags that AA seems to let people bring into the cabin and 2) as US resident you can get AA credit card for cheaper for the same benefit (+ bonus AA points which we know is superior to AM).

So in essence one is paying USD 110 for CX lounge access + CX seat assignment for the one (or maybe two) time flying CX. As percysmith mentioned above that's a lot of dandan mian ...