I think the rubber part is for noise reduction. Most replacement sprockets I get do not have it and no issues.
I usually run the stock gearing which I guess is 15/45. I have tried 16/45 but that was too high for off road. 14/45 is too low for all the highway I have to do to get to the dirt. For 95% of the stuff I ride off road the stock gearing is OK, but I do not ride much single track.
2009 KTM 690r Enduro SOLD
2002 RC51 SOLD
2010 KTM 990 Supermoto R SOLD
2014 KTM EXC 500