You are definiitely correct on our cars wearing out from years of use and abuse. Carpets take a beating no matter how hard you try and protect them. Even more-so, the underlying materials that take the brunt of our cleaning products too. The mass back loaded carpets are nice for the basic restoration. Mostly your decision will be based on use and future goals for your foxbody. What's your overall agenda and purpose for the car? Attack this area just like you would the performance side of your build. You wouldn't hopefully put cheap parts in the motor, transmission, and
suspension of your ride. Treat your interior the same. I see you're looking at the Noico line of products on Amazon. While effective, Noico is not the best out there, even for the price tag. Noico is a asphalt based sounddeadener. Depending on your location, and sun exposure, you may not want the smell of asphalt to always be present in your car. Unless you drive with the windows down all the time. You want to find butyl based deadeners. The greater the thickness, the more it will reduce noise, vibration, and increase insulation. Most products now range from 50-80 mil thickness. Most common household name is always DynaMat. Their lower end products for soundeadening are okay, but asphalt "rubberized" based constaints. Still going to fail at some point. Just not as fast as the Noico product. My top two picks out there were sounddeadenershowdown and secondskin. Don over at sds was a great and knowledgable guy. Gave great advice, and charged hardly nothing for his superior product. Sadly he has closed his doors forever. My second, but first choice, is Secondskin. Great product, with great reviews, and customer service. Check these guys out when you get a chance. As for your questions:
Is the mass backing worth it? Depending on how you want your interior to sound and feel, definitely. Can you save some coin, and go with a nice low cut pile, also yes.
Does using sound deadener and standard carpet yield better results? Depending on the products used, and the orientation of those products, Yes.
Is sound deadener worth it? Most definitely. But again, how do you plan to use your foxbody? Weekend warrior? Cruiser? What's her prupose? I ask cause in all reality you're putting weight back into the car. Negating any weight saving s you may have done previously. So only you can decide if it really is worth it.
What's everyone using for sound deadener these days? I see some cheap options on amazon. Fore go the cheap options if this is a keeper foxbody. If you plan to keep this car for the rest of its years definitly do not cheap out. If this is just a for now project, then do as you please. Heck why even dig into this project for a short enjoyment? Like stated above I chose SecondSkin. Their CCF, MLV, CLD tiles are great product lines. They have more options, but these are the main components in my car. I'm shootng for a whisper quiet inteior. I want to be able to have a conversation with someone on the HotRod Power Tour, and not have to scream over the engine noise or radio. Even have a phone call conversation without all the wind noise too. In the pic I used Spectrum in the 5 gallon bucket. It's basicallly bedliner material and Por 15 put together. You can spray or lather it on. You only need about a 2 mil layer with this product. I did core support to rear light panel. The boxes laying down are their butyl based soundeadening mats. These are their Damplifier Pro pieces. These were applied to the car's interior sheet metal. From firewall to rear light panel. This sandwiched the floor and rear trunk fully. The metal has no room to vibrate or let outside road noise in. In the standing box is their MLV roll, Luxury Liner Pro. It's a MLV and CCF mated together. It will increase the noise barrier a substantial amount too. This is just the floor pieces. Doors, trunk, trunk lid, pillars, and roof are done using some of these and other products from SeconSkin. Just the flooring alone was a world of difference. Now with the rebuild, I can take it further.