Regardless of what you do to the engine, your enemy will be the hull. All hulls are built for a function, ours are built for a wake at 36 mph. Most Outbacks are topping out at @45 mph (give or take 1-2 mph). As speed increases, so does the friction on the hull (per the way it is designed). Check the Nautique forums for speed issues. Trying to break 55mph in an Outback would probably require more than a turbo and may be nearly impossible.

Even the Python (496/502 cid) motors used by CC in 196 were only in the low to mid 50s.