Wolly,
Heat Pumps have the ability to heat and cool, such as here.
http://aquacal.com/heat-pump-informatio ... index.html
The amount of time needed to heat up your spa will be proportional to what size heater you currently have. If you have a 400,000 BTU heater, at 70% efficiency (optimistic with a 25 yr old gas heater), you're getting 280,000 BTU output of heat. The AquaCal SQ156, for instance, provides 127,000 BTUs of heat, which is a little less than half the BTU output of a 400,000 gas heater. So, it will take a little more than twice the time it took for the gas heater to get your spa up.
If it took 15 minutes with the 400,000 BTU gas heater, it will take a little over 30 minutes for the heat pump to do it.
That's not too bad or much of a problem. Just switch on the heater 30 minutes before you want to use it, rather than 15 minutes before.
In Houston, the temperatures really don't drop down enough to warrant using the gas and heat pump approach. During the months that you're more inclined to venture outdoors and use your spa, are the same months that a heat pump would be very efficient in maintaining heat to your pool.