How to set paper thickness on Pixma ix6850

I bought the ix6850 because the specs said it could take up to 300gsm thickness. When I try to print on 300gsm sheets using the plain paper setting, I keep getting spots of ink. When I choose Photo Paper Platinum Pro which is 300gsm, printing is too slow. Is there a way to set paper thickness without using the slow presets and use the paper option instead which is much faster? Or is there a way to speed up the printing for the photo papers? What I'm printing is quite simple, I just need it to print fast and without spots. Thanks.