Always X8 burn speed, Old versions of Toast showed that was the highest for my old fully spec'd iMac.
A disc burner and CDRs might be up to x52 but the computer is rarely !!
Mine works at X52, but Im not so impatient that I need it at that
X8 always works and even with my Macs (see below) I never go higher
A good way to check a burned CD is put it in a CD player, press play, then ffwd thru so you can hear it sped up, if there is any stuttering on the counter as it goes , Its been done too fast
If it forwards and the counter is smooth as it displays time, its fine
I store burning up so I can set stuff going while Im watching TV, not staring at the screen going "come on burn burn burn"
Then theres no need to rush and risk having a load of coasters