8MHz of what? Bus clock? Oscilator clock?
You have PMPRCLK=0 (1:1 prescaler), PWMCAE=$80 (center aligned), PWMCLK=$80 (clock SB), PWMSCLB=0 (SB clock is clock B /512), PWMPER=$FF (PWM period = $FF=255 SB clocks). I think ith these settings and 8MHz bus clock you should get 8MHz / 2 / 512 / 255 = 30.6Hz PWM period. Where first /2 divider is due to center aligned mode.
I don't know why do you get 2.75kHz. Setting PWMSCLKB=0 (clock SB = clock B /2) and PWMPER=181 you should get
quite close to 11.05kHz PWM period, of course if your bus clock is really 8MHz. In this case you get less than 8bits PM resolution, duty cycle setting (PWMDTY) is limited from 0 to 181. To get the best resolution use edge aligned mode, no prescaler and concatenated (16bits) mode. PWMDTY67 = 8MHz / 11.05kHz = 724