I hope you are well.
I had a Promax 300 on my Phantom 25 for 7 years, and did have a lower unit breakdown, powerhead failure, etc. Rerigged to a twin 2002 Merc 2.5 EFI SS with Brucato PCU boxes. Absurd enough the engines idles better (due to the the PCU's) and the fuel consumption is up only 20% compared to one 3 litre engine. What suprised me was the power of these engines, pitch and sound. Great fun.
Micheal Werner, a 6 time world champion in F1, told me: The promax 300 is a lazy fishing machine with a nice cowling and a bad ass midsection. The 2.5 EFI is a strong engine if taken care of. Its the ultimate outboard. Its a very simple design.
However, they need good synthetic oil, good quality gas and regular maintance. According to JSRE you can run them 140 hrs each before tear down. Check the leakdown once in a while, and clean the injectors each spring.
A friend of mine swapped from 2 x 2.5 EFI engines to 2 x 250XS engines. They use less fuel, they smell less and they a more quiet. However, the engines have been teared down three times in 2 years.
I have decided to keep my 2.5 EFI engines. I have began to learn them. By careful simple maintaince, they will run like hell day out and day in.