The reason for the use of such a high explosive load, including the SAP bombs, was that most of the 'burnable buildings' in Essen were already considered to be destroyed by earlier raids.
The raid in question involved 1,055 aircraft, dropping 4,538 tons of bombs, 90% high explosive.
The local authorities in Essen reported 607 buildings destroyed, 812 seriously damaged, 662 people killed and 569 injured.
Bomber Command lost 5 Lancasters and 3 Halifaxes.
It is a sign of the state of the air war that Bomber Command could put more than 1,000 aircraft over Essen, without the participation of 5 Group!
Cheers
Steve
Edit: To put that 90% HE in perspective, typical area raids on cities would deliver an overall load which was between 40% and 50% incendiary by weight.