Hello

yes it is a lot beneficial if you do MBA further MBA in finance would be perfect and would go along with your field, other fields that you can go for are MBA in Marketing, MBA in Human Resource Management,MBA in International Buisness.

You must atleast score atleast 95+ percentile in CAT to get a top IIM.

to know more about CAT check this link: https://bschool.careers360.com/exams/cat

Absolutely every course is valuable. Every subject has its own importance. Moreover you are studying maths so you must be very good with numbers which will be helpful both in CAT and in your management studies as these days analytics is playing a very important role in the industry. 

If you get a percentile of 85-90 in CAT and you have a good profile you can get an IIM call. But it's always good to score more. 

All the very best