Does the 815 non-TG have the TG N-connector in place?? Oddly the only picture I've seen has been with 2 N-connectors at the front. If so it certainly looks promising for a "software-upgrade".
For those who has dismembered the 815... how about measure the IF filter size, this way we could estimate the IF frequency, and thus see if the 815 is hardware designed for a higher than 1,5 GHz stop frequency. Or even better try to measure the cut-off frequency of the first low-pass filter. But this may be a though job.