Throw out the bid that wants to install a 3 ton a/c on a furnace that only has the capacity to move 2 tons of air. You currently have a 1.5 ton unit. You cannot double this size. Not only is it not warranted, but your ductwork would not handle the extra airflow. Looks like the quick 500 sqft per ton rule, which is highly inaccurate. The 15 SEER heat pump would only be 15 SEER (or close to it) if the new furnace also had a variable speed fan motor. I think for others to advise you on the payback/savings part of your question, they'll need to know your current gas and electric rates.

Here in Cent Fl, the best combination, in my opinion, is a 13 SEER heat pump with a heat recovery unit. The HRU will raise the effective SEER to about 15. The equipment savings on the lesser efficient heat pump will more than pay for the HRU and you get free hot water from then on. The claim is that the higher efficiency heat pumps (14 SEER +) don't generate enough heat at the compressor for the HRU to function properly although I haven't been able to document that with instrumentation. Also, a reasonably tight house may not benefit from the ability to slow down the fan speed for dehumidification since a properly sized unit will probably maintain a comfortable level anyway. The cost of repairing a vari-speed fan after warranty is horrible...and the fans generally have only a 1 year as opposed to the longer warranty on the condensing unit. I've seen many more vari-speeds fail than standard fans. The bill is usually over $1000 as opposed to several hundred. The combination of a 13 SEER with HRU is providing a $130 monthly electric bill in our new 2000 sq ft house keeping the temp at 76.

On the HPs, ask for upgrade price on the Performance series 25HPA5 sgl stg and 25HPA6 two stage HP condenser. You did not mention the thermostat or controller. It should be the Infinity model. IMO

“Another consideration regarding a HP is that you might have to increase the size of your electrical entrance.” “Re the need to possibly increase electrical service, I don't understand that.” If heat strips are required, the electrical panel might have to be upgraded. If needed, the existing electrical capacity might not be enough. I do understand the climate of the Pacific North West. “… No need for a HP in a DF system” It depends where the installation is geographically. Again, I do understand the climate of the Pacific North West. If DF means ‘Dual Fuel’, that usually means a fossil fuel and electricity, which exhibits its greatest efficiency when powering a heat pump. Hybrid systems (gas & heat pump) can have some of the lowest operating costs that can offset a higher installation cost – even in residential applications; again, depending on geographic location. IMPO SR