I have a Westerbeke 52 with only 1,350 hours on it in 20 years (and 20 winterizations, 18 of which I didn't perform). We are planning a circ in 2-3 years and I am having the engine rebuilt this winter (or rather, gone over with a fine-toothed comb to see if anything needs rebuilding), because I may put on 1,000 hours a year for the next five years on a block that's seen maybe 80 a year until now. It is worth it for me to replace early and to replace injectors, heat exchanger, lift pump, gaskets, etc. now than halfway through our trip. By the end of our trip, we could have those 8,000 hours you have now. I want a fresh start, and I want everything that is working today (because the engine works fine) as spares for the trip. I have had some success with the "early retirement" of bits and pieces that have become spares and find, for instance, the bolting on of entire engine parts (like pumps) easier at sea than servicing them...that can wait for the next anchorage.
So I guess the answer is simple: would you rather rebuild here or in Fiji? 8,000 hours is well into middle age...do it here and now.