I guess I should have prefaced this post with a lot of information for those who don't know or choose to forget.
This mod is not for everyone. It's for ME.
1) I autocross this car regularly. (at least twice per month, EVERY month, year-round) I've had to make some significant (yet fairly simple) modifications to make the car handle better. And I'm having an absolute blast making my 3-cylinder econobox go around courses faster than much "better" cars that cost 2, 3, 4... and 10x as much and have way more power and acceleration. I've been doing this long enough to know that "it's more the driver than it is the car", but it's still fun to see people sneer at the Mirage... and then the look on their face when they can't touch it!
2) I like lightweight cars, but I can't justify spending $40,000 on a Lotus. And that's why I bought a 2000 pound car for $6800. That car just happens to be a Mirage because it's the lightest "new" car on the market these days. And I seek inexpensive gains in power and reductions in weight where I can.
3) I live in Florida. We don't need huge batteries for reliable winter starting. The reason the Mirage has such a large battery is just that. They want all-weather reliability for all buyers.
4) I DO daily drive this car... and my wife rides in it with me a lot. So it needs to reliably start (thus I'm not removing the battery entirely). It also needs to be relatively quiet, free of rattles and squeaks, have a full interior, and functional air conditioning. Were it not for all of that, I'd still be driving a 1976 Triumph Spitfire... which would be none of those things!
5) I have AAA towing coverage if I need it!
I'm not suggesting that ANYONE do this particular mod, or any other mod that I've done. If you happen to share the same goals as I do, then you might be interested in them. Most people don't autocross a Mirage. It's an economy car, and that's what most people buy them for. I understand that. You need to understand if you're reading my posts that I am not the typical economy car owner.
To be clear, I'm not just doing this stuff "because". I'm doing it specifically to make the car perform better on an autocross course. Weight is king. In the world of motorsports, reducing the weight improves everything about a car.
Lithium Batteries - They are INCREDIBLE technology. I could have gotten my battery weight from 6 pounds down to about 1.8. And instead of $32, it would have cost right about $100 or more. So, the last 4 pounds would have cost me $70. Not quite worth it. Especially when you consider how fickle and unforgiving Lithium batteries are. I ran a 2-pound Ballistic battery in a Miata for a while. It worked GREAT! The amount of amps that little battery put out was amazing! And then I left the lights on once and drained it below its critical threshold of 9.5 volts. It would no longer fully charge or hold a charge. It was done. No thanks. That's why I went AGM this time... though I did do the research and consider it for way too long... it's just so darned COOL! But, the $32 motorcycle battery makes a lot more sense.