It's both YES and NO. I don't think you can get the true answer directly from any sennheiser representative, of course they will tell you their stock ear tips are not required.
But once you try enough 3rd party ear tips on the IEM, you'll realise none other than the stock ear tips give the same sound.
What's really change the sound here is not the ear tips. It's that little piece of black foam inside the tube of the stock ear tips doing the job here.
It doesn't matter what ear tips you use as long as the tube size is the same as stock, the black foam is inside and fully cover the tube it will sound the same, but there is no way to fix a piece of black foam inside the tube of any 3rd party ear tips, you'll need special glue for silicone otherwise it just won't stick.
So the stock ear tips is the only answer, you have no choice if you want the exact same sound of the stock ear tips.
Use the stock memory foam ear tips, it is a little bit bigger than the silicone ear tips of the same size.