Can I get the 250K baud at this freq?

well, let's see.

assuming you can preserve R0 as buffer pointer only and your receive buffer is in (I)DATA memory. assuming it is receive only

ISR:
push a       2
mov a,sbuf   2
mov@r0,a     1
inc r0       1
pop a        2
reti         2

for a total of 10 cycles
then there is interrupt overhead at max 5 cycles for a total of 15

assuming a 12 clocker you will need 12*15 = 180 clocks

so - to avoid calculating - at 18 MHz you can handle 100k BYTES which approx is 1000k BAUD.

The above ISR is NOT recommended - it relies on a register not being used elsewhere BAD practice, but it does show a) how to find out and b) that it can be done.

Erik