What are the BTU/hour heating & cooling specs for both the old and new systems? (Or the model numbers, so's I can look 'em up.)
Almost all heating systems in the US are more than 2x oversized for the actual loads, in part to make up for duct inefficiencies. It's worth using designing the ducts to Manual-D spec, using hard-piped ductwork and mastic-sealing every joint & seam, and even FSK-taping the seams on the air handler cabinets, then "right sizing" the mechanical systems to the actual loads. With ducted-air systems this is more of a comfort issue than an efficiency issue, but at 5x oversizing even 2-stage gas furnaces start slipping down the steep part of the efficiency regression curves.
If you haven't already, start buy doing a room by room heat loss calculation based on realistic indoor temp and the
99% outdoor design temp for your area, the window type/U-factors, wall & attic types & R-values, etc. I can show you how to do a quick & dirty I=B=R method estimate using spreadsheet tools if you like.
I can't tell you how many time's I've seen 150KBTU/hr furnaces & boilers in houses with 30KBTU/hr heat loads- it's rampant! With 5x oversizing they are theoretically good down to about absolute zero as an outdoor air temp- what's with that? Hopefully yours isn't that gross, but even 2x oversizing is a comfort-mistake. AFUE is tested at 1.7x oversizing, and beyond that furnaces start slipping a bit on efficiency (except for 2-stage or modulating condensing furnaces.)
BTW: Since this is slab-on-grade, are the ducts & air handler in an attic, above the insulation? If yes, controlling air leakage on the ducts is CRITICAL, since leakage would then drive air-infiltration losses for the house (quite literally) through the roof! Controlling the air leakage at the attic floor/conditioned space plane is also huge. On houses like that when it's time to re-roof, consider putting 2.5-3" of rigid foam above the roof deck and 5-6" of open-cell foam on the underside of the roof deck, sealing off all attic venting (and blower-door test it before the foam guys break down). This puts the ducts & air handler fully inside the thermal & pressure boundary of the house, typically gaining more than 25% on system-efficiency, and even more on comfort levels (particularly on muggy mid-summer days.) It's not a cheap solution, but it's worth it on comfort, even if the payback on utility savings is long.
Timers/ programmable thermostats on water heaters save next to nothing, since to save anything on standby loss the temperature in the tank has to actually drop. The only way to save substantially that way is if you turn the water heater off THEN take a shower or fill a tub, leaving the tank at a much lower temperature until you're ready to heat it back up again.
Even at cheap gas rates it's worth insulating all of the hot water distribution plumbing, and then near-tank cold-feed & and T&P overflow plumbing to R4 or so with closed cell foam pipe insulation. This reduces standby loss measurably, and extends the amount of time the already-drawn water in the distribution plumbing remains warm enough to be useful. Most of the foam pipe insulation at the box stores is 3/8" wall R2. Some TrueValue stores will carry 5/8" wall R4 suitable for half-inch plumbing, and Grainger has a wide variety available. Otherwise you may have to go through a local plumbing supply house or internet source and buy a box of it. But the 5/8"-3/4" wall goods is worth seeking out.
See:
http://www.leaningpinesoftware.com/hot_water_pipes_pipe_cooling.shtml