There is a still one possible obstacle to move from +/-15V to +/-5V. Down-programmer circuit require negative voltage which has to be a little bit over -5V or logic level p-ch mosfet for Q5 has to be used. Unfortunately it seems that such device is a rare animal in TO-220 package and at least -55V. Until now I found just a few: IRF's IRLIB9343 or Vishay's SUP53P06-20 and SUP90P06-09L. Another possibility is to extend input power connector for another two pins and bring -8V from pre-regulator that is used for -5V LDO.
Ah, but it's not hard switching, is it? And it doesn't much matter if it does, as long as it's able to pull the output reasonably low with enough current (100mA? amperes?). All of these seem to be overly large, in fact! Though you might have to simply roll with it, to get the power dissipation.
I'd avoid the IRF part: just at first glance, Vgs(th) is not in a guaranteed range, and that's no good for a linear or semi-linear sort of application. The SUP53 seems nice, and is also guaranteed for switching at Vgs = -4.5V, so you know without a doubt it's more than overkill. The '90 is most likely a scaled up version, so unless you need the sheer switching capacity, you're just paying more for no benefit. You can probably go much smaller, if availability and dissipation are still there.
Board layout looks generally good. You've got good conservation of ground continuity and all that, I see more use of vias, and the main current paths are either routed closely, or bypassed nearby (in the case of the power supplies).
A note: I see the ground "jumpers" you've placed, but they're used a little inconsistently: that is, the longest bottom side trace has one jumper over it, splitting it into two spans...which in turn still have lengths several times of any other slots!
If you are limited to one jumper, placing it towards the middle is probably the best idea (which I think has been followed), but if not limited -- the more, the merrier.
The purpose is partly to address the inductance (or the RF resonances) of the slots, but equivalently as well, the sources that might induce currents into those slots -- namely, any top side traces crossing them. To keep current loops smallest, the jumper(s) should be placed alongside the most sensitive, noisy or high-power traces that cross the slot. So you're not only shorting across the slot (making it a shorter electrical length), but providing a current path, directly where it's needed, preventing that offending current from inducing currents elsewhere in the first place.
I'd also suggest using wider copper (in most cases I think you have enough space to go at, say, the via OD?), since the main hazard to a power supply application will be low impedance ground loop currents, and a thin trace isn't much better than a slotted pour, even if it's pretty deep.
Tim