I've been doing a lot of digging around for info like this, as I'm ready to repower my 5.4 Searider. For DI 2-stokes, your options are pretty limited: Evinrude E-TEC, Mercury Optimax or Tohatsu/Nissan TLDI. I beleive the E-TEC is a bit more efficient than the Tohatsu. I wrote off the Opti immediately, as it is a porker, quite heavy compared to the other two. That's the same reason I'm not interested in a 4-stroke.
Most of the user reports I've found, indicate an improvement in cruising range/efficiency on the order of 35%-50% depending on use for folks repowering from a carbed 2-stroke to an equivilent E-TEC.
It's tough (nearly impossible) to find a 2-4 year old used one of any of these. I've seen a few E-TECs on eBay (USA) but they sell at a premium, not enough of a discount to justify the lack of warranty IMHO. I've been searching and pondering for several months, and I'll be laying down the cash for a new 90 E-TEC in a few days.
Carbed 2-strokes, 4-6 years old, are easy to find as they're frequently being replaced by newer tech engines (DI 2-stroke or 4-stroke)