I recently purchased a 1998 25 outlaw. Which I am putting a 525 efi in. My question is which lower unit will give me the most top end out of the boat. I have a stock XR or an Imco -1 shortie in which I could use.

I would try the Imco first to see if it runs without any issues like getting on plane or prop cavitation problems. You can always go back to the stock XR lower if it doesn't work.

I had a 25 outlaw with imco drive with a -1 shorty and the boat liked it.