Extension 1 is no different from Advanced except in a few select respects:
1. Essays are significantly longer (about twice as long as a standard Advanced extended response, although you only do two as opposed to five)
2. Marked slightly harder, although generally on less juvenile criteria (whether your material adds up is more important than your structure and grammar)
The teaching style is very similar to Module B of Advanced English (Critical study of text, expectation that you will integrate sophisticated analytical readings into your essays).
If you're doing well in Advanced, it's extremely likely you'll get a similar mark in Extension 1 - the scaling compensates for harder marking.