I haven't used an 8XL, but assuming that you were to use a lower gear ratio with the 8XL than with the 7XL, the power difference shouldn't be too noticeable.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz