First of all if you, change the tires from stock, you do get not more than 10% grip (best possible), which at the cost of 50 to 70 % increased fuel consumption because more power would wasted just to spin the tyre..... so take it this way, you lose money on bigger tyres (bigger than stock), lose some more money on fuel daily, for just 10% more grip.... that's totally not good spending at all. moreover as the company provides you the tires, they are no doubt given after tons of calculations, never think that the company people are stupid and should had given bigger tyres. So even when changing tyres buy exactly the same sized as provided by company. and the real deal that matters the tread design that really plays a big role.... just that if there is euro star from general having size 195/65/R15 and there is Toyo Proxes in the same tyre size that drive quality will be entirely different. so if you put it in simple words go for the most expensive tyre you can buy in the same tyre size....
for reference check 1010tires.com, calculate your tire size there for getting an idea that how much big will the tire will be.
and the for the psi you have asked i really can't figure out the correct tyre pressure.
The Mercedes C63 AMG Experiment - /CHRIS HARRIS ON CARS - YouTube check this out....