The swap would be a bandaid, nothing more... You still wouldn't have fixed the real problem. Unless you've got the heater running on full and the headlights on you shouldn't see it dim significantly when you step on the brakes... a little isn't surprising. Start with the basics first (It's MUCH cheaper too). Regardless of the voltage you're seeing you should take the battery to a shop for a complete battery test (charge/discharge); It takes about an hour so you'll need to drop it off. There are home chargers which have testers built in, the method and accuracy with which they test varies significantly so I would read reviews and don't cheap out. IMPORTANT!!! If you have an AGM (Absorbed Glass Mat) battery do NOT connect it to any charger which does not specifically have a setting for AGM!
The only time I really recommend a larger alternator is if the electrical load has been significantly increased placing demands on the charging system well beyond the original specs; Huge amplifier, electrical fan conversion, electrical cabin heater or an old battery with a bad cell or two. The only other time I'd consider it is if it's a city car where it sees lots of short trips and the battery probably isn't charging enough.... My wife goes through about a battery a year if I don't keep on it with a charger every few weeks.
No, the MR-2 isn't a straight swap in but I think it was close... Something like swapping the pulleys, maybe the wiring pigtail and a washer.
I highly suggest you fix the real problem though. A larger alternator still may not remedy the symptoms.
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