I just yanked the 165HP Perkins out of my boat. (Hydrolocked- due to a weird
failure in the intercooler- and with 4000 hours on a 20 year old engine it
seemed the right thing to do).

I replaced it with a naturally aspirated Perkins, somewhat the same engine but
with a better injection sytem and an exhaust manifold cooled with the engine
coolant rather than salt water. The NA version of the engine is 135HP.

I thought I'd probably lose a little speed, but I've picked up at least a half
knot.
(I do cruise the NA 100 RPM higher than
I did the turbo)

One of our local Perkins mechanics has told me, in the past, that he has been
able to convert the 165's to naturals.
I'm not sure what the process involves, but he led me to believe the engine
will still run
OK with the turbo removed. (It would make sense that the injectors and/or pump
might need to be changed.) At the lower end of the RPM range you report, you're
only getting "some" benefit from the turbo in any case.

I wouldn't worry much about overheating the engine due to lack of an
intercooler if you switch to NA- the air charge cooler is really only required
because the air gets
superheated when it is pressurized by the turbo. I'm not sure the air on the
output side of the air charge cooler could ever be considered refrigerated. :-)