I have achieved frequency stability to 1 Hz with the Flex-1500 & Elecraft XV144 transverter combination, using a GPS disciplined OCXO on 116 MHz. I have documented the XV144 frequency stability and drift data comparing the GPS control against the stock crystal oven option with enhanced thermal insulation herehttps://picasaweb.google.com/114032640816757126398/XV144FrequencyStabilityModifications
The stock FLEX-1500 TCXO is sufficient to hold the frequency to within 1 Hz. Enabling the 10 MHz sine wave external reference on the FLEX-1500 should increase the frequency accuracy and stability to the millihertz range.
The GPS reference we used was engineered by N6GN specifically for members of our 2 meter WSPR study group listed on http://www.qsl.net/kp4md/144_mhz_wspr.htm
That discussion thread can be followed athttps://groups.google.com/forum/#
A ready made solution to produce a 116 MHz local oscillator locked to a 10 MHz GPS disciplined reference is N5AC's VHF ApolLO at http://www.n5ac.com/blog/products-page/apollo-synthesizers/vhf-apollo-assembled-tested/