I would like to think that all of his strength is purely his own and not an extension of his suits abilities. However I understand your concern. I don't want people ever thinking something along the lines as "Superman wouldn't even be that powerful if it wasn't for his suit, blah blah blah"
I do remember though that at the beginning of Superman Issue 13, Superman was benchpressing about 6 Sextillion Metric Tons for 5 days and he only broke a single drop of sweat. He wasn't wearing his suit during these trials.