DStv does not transmit on the W4 African beam, nor on the W7 South African beam, but only on the W7 Sub-Sahara Africa beam. (See http://www.lyngsat.com/ew7.html
As you know this beam is on the KU band, and it's the only subscription beam you will have access to.
If you haven't already, you would need to subscribe via the Madagascar Multichoice as a resident to gain access to this beam's content ( http://www.dstvafrica.com/dstvafrica...adagascar_home
Unfortunately the true DStv South African beam, from IS7 ( http://www.lyngsat.com/intel7.html
) only just covers some of the Northwest edge of Madigascar ( http://www.intelsat.com/flash/covera...lay=map&spot=4
The only other DStv related beam available (may be temporary) in Madagascar etc. would be the one you mention from IS10 (roughly same location as IS7) on C band, but this is not available for normal subscription and consists of limited channels for special usage.
(The original IS10 C band option was scrapped many months ago.)
A larger dish will always help improve signal reliability, but best to consult with an expert in your area.