I don't think Sanders would get 80 seats. A Sanders leadership with a similar style of lunacy - Your car breaks down. Who do you blame? BANKERS! - and inexperience that we saw in the Social Democrat lot would probably anger a lot of prominent moderate Labour people and the party's moderate voters. This anger could then cause the moderates to break away like how David Owen and Roy Jenkins did when Michael 'Worzel Gummidge' Foot was leader of the UK's Labour Party IOTL.
If it was Sanders, Leader of Labour, instead of Sanders, Leader of the Social Democrats, that was saying that, I doubt Sanders' Labour would do much better than his SDP.