I’ve never driven an l251.
Mild na ejde 70hp without ridiculous cost. But some time on head, headers/exhaust, bigger throttle plate, ecu crystal to increase redline or aftermarket ecu, and vernier cam wheel or cams.
Turbo EJ-DE, std block and head with custom manifolds, after market ecu very reliable 130fwhp. 150fwhp a bit problematic. Some will argue the CB80 is better. It is older tech. Parts are rare. It is bigger. Having cut EJ-DE heads in half the cross section is akin to what Cosworth do with downdraft inlet ports, narrow valve angles and good combustion squish. There are stories CBs with 250hp, unconfirmed. Somewhere on [https://www.autospeed.com.au] is the story of a race TRXX awd wtih CB80 that had a huge amount of development and appeared to be no expense spared (although they kept the factory gearbox which was the strangest thing as it would be the weak link). It was only something like 180hp and not sure if crank or wheels. Maybe a CB80 is “better” but I don’t see many and the one time I did look at buying a pair of engines I decided they would be a money pit.