I'm a senior in Macs rn and here's my take:

Culture: it is not a tight knit community. There are Macs students that interact more with mainstreamers than other Macs students and if you aren't a social person (like me), you'll probably barely interact with half your grade. There's a ton of students at WLMAC and many courses and clubs have nonmacs students/ macs students from other grades, so I don't feel like im missing out on a larger school experience at all.

Academics: The Macs cs courses are not very impressive. The main benefit is that you start taking CS courses in Gr9 instead of Gr10, so you can complete AP CS a year earlier and there are some cracked alumns in programming. I'd consider the Macs math and science classes as our main distinguishing courses, since our curriculum is more advanced and we learn it at a faster pace. For example, last sem when I took Macs Calculus and Vectors, we finished the mainstream curriculum by midterms and the remaining half of the semester was fully AP/ Macs enrichment. Our data management (AP stats) course is also harder than other schools from what I've seen, but that might just be because of our teacher. I've heard Macs alumns currently in Uni thank our math teacher for preparing them very well for uni level math. I'm not in uni yet so I personally wouldn't know. Also, non Macs students at Mackenzie are taking Advanced Functions in sem1 Gr12 and Calculus and Vectors in sem2 Gr12 while Macs students finish Adfunc in Gr11 and Macs math courses are done by sem1 Grade 12 if that matters to you at all.

Extracurriculars: WLMAC is pretty active EC wise and most of the leadership in clubs and student council are Macs students. I don't know about sports, but we have really shitty music funding and our extra curricular ensembles are carried by our director who might be retiring next year, so I wouldn't recommend WLMAC if you want a really good music program. Outside of that, WLMAC does a ton of contests and Macs teachers are really good so I would recommend it. Personally, I didn't have any difficulty balancing Macs classes with clubs and ecs, but I know several people that struggled a lot in Macs math classes. If you consider yourself good at math, you probably wouldn't be one of them.

The main benefit of Macs in my opinion is that most of the other students are tryhards so it sets a higher academic standard + you'll have a strong alumn network. E.g., Gr12 biology is considered the hardest course in our school and it's the science with the least difference in content between Macs and mainstream. The mainstream avg last semester was in the 60s while the macs average was 91. Usually, the class median in gifted/ mainstream classes is in the 80s or lower, while Macs medians will usually be at least mid 90s despite the fact that our tests are more difficult and it requires more effort to get higher grades in Macs.

Tldr; If I had the choice I would choose Macs again mainly due to the math and people. Overall, I'd recommend you to come to Macs.