KW "Was that -3's or -10's looks like -3's to me with 3 bladers.
This seems to be the crux of the performance argument, both have (had) three blade props.. Looking back at old PFR the – TPE 331-3 (three blades) was comfortable at 245/250 range TAS, averaged 560 pph in the F17 – F200 range; with the TPE 331-10 the TAS was higher 265 (ish) for about the same FBO. The big advantage of the -10 was to be able to kick it up over the F200 barrier and stay out of the icing layers. The -3 engines ran out of puff and with the engine anti ice on, descent was often the only answer to ice.
One of the turbine operating norms is the control of SGR, by selecting the TAS, Flow and Altitude to best suit the weather conditions. For example LRC @ A100 TAS 232 Flow 610 v F200 TAS 242 Flow 510. The 'Mark' of the Merlin engine doesn't matter if you use SGR for planning, as you decide the Flow and accept the TAS; or select a TAS and accept the Flow; it depends on the day and what you need to achieve. This does not apply for chuffing about the place on short 2/ 300 miles sectors, unless you are load critical, then it's handy.
On long flights, effective TAS needs to be considered; say you were looking at heading West, with a big NW wind; the 'crosswind' component reduces your effective TAS, however it can be advantageous to go high if the wind is nearly at a constant negative 'value' (TAS v ETAS v Flow); increased TAS, decreased burn off; but you do need to evaluate which is the 'best' height for the job. One of the great thing about the Merlin is the speed range at 'low' level; set up 600 pph @ A100 and watch the speed, same at F150 same flow better TAS, but knock the flow down to 550 or even 500 and the speed will not diminish greatly (weight). With minimum fuel and maximum payloads, the SGR becomes important.
Looking at HLB figures I'd say, without knowing the aircraft they are a little light on; 260 to 270 is about 10-15 knots over the odds for the average -3 and in ferry configuration (weight) at F200 a 500 pph flow seems a little low, but not too much, perhaps it's a fast ship. For average – 3 Merlin (MGW) planning 245 knots 600 pph F 200 is as safe as houses, too much above 200 and the TAS slides away simply for lack of power, so it all depends on the demon wind. But full tanks + 269 USG would be enough and Snowgoose is no goose.... Just saying...