There was a thread on a homesteading forum I follow where a bunch of old timers argued about this for pages. One guy who processed hundreds of birds a year said he switched to a sawzall with a fine tooth blade and never looked back. He claimed the key is letting the tool do the work instead of forcing it, and keeping the blade cool so it does not gum up with fat. A few others swore by their old handsaws and said power tools chew up the bone and leave fragments. The thing that stuck with me was a guy saying it comes down to how many birds you do at once, since one or two a year does not really need the upgrade. For somebody doing a bunch like you seem to, the switch makes total sense.

Did you check the cat before throwing parts at it? Those old Miatas are known for burning a little oil, and oil kills the cat over time. Techron helps with injector gunk but if the cat is on its way out you're just masking the problem for one test. The freeway run works because you're getting the cat nice and hot, that's the real trick there. Next time get it good and warm, no sitting in line with the engine off, and keep it in a lower gear on the way over.