My experience with the Monza systems is rust and NOISE: they will rust like nobody's biz in moist climates, especially if the car isn't stored in a relatively tight garage.
Also, they are a long way from the OEM or stock appearance. Looks more like something that should be on a Ferrari or race car than a stock LBC, IMHO.
Also, my GT6 came from the PO with a brand new Monza on it, and it was LOUD. If you like LOUD, they're fine. It was so loud that at highway speeds it would render headaches after about 30 minutes. I like a "sporty" sound, but not at the expense of a headache and tinnitus.
If you're into stock, most of these cars (MGs, Triumphs, etc.) were pretty quiet from the factory. The factory exhaust system on a GT6 or an MGB, for example, rendered a very quiet car. So quiet, in fact, that many folks opted for the Ansa or similar systems to get more noise.
YMMV. But remember, the Monza system is inexpensive, but it will rust quite quickly.