If the decks are the issue then it kinda sounds like your stuck... Don't know much about the 135, we looked at them but at least with the 109 you will sit at 140 indicated generaly around 75-80%. It is possible to save considerable fuel by sitting at 5000 and achieving 160 tas.
The 109 is a little on the heavy side empty doesn't leave you with much, full fuel and 3 pob is a squeeze. The standard max t/o weight being 2850 kg, perhaps go for the 3 tonne kit that will make it a much better option, the machine will do it no problems...plenty to spare, just a bit exie for the mod.
The pratts have proven reliable and perform very well, generaly you don't touch em, just check the oil and wash off the soot.
Sorry don't know what floats add to the equation.
Thats it from me good luck.
P.S Shame that all that extra stuff is required.
Oh and 2 pilots and 4 pax, gives you 392 kg of fuel, 30 min res, and with a tas of 145kts you should cover 188nm.
Thats using a standard burn rate of 220kg/hr, I would imagine you could easily better those numbers on the day.
I think the grand is going to be around 3300 kgs mtow and would think not much heaviel that the power...better again.