Ya, I had 750lb progressive springs and adjustable struts on my 89 hatch and it was a kidney buster. I'd like to know this spring rate too.
Keep in mind that when comparing different cars, the spring rate doesn't say much if you don't take the leverage of the control arms into account (motion ratio = inch of spring compression per inch of wheel movement). With the motion ratio known, you can calculate the wheel rate by multiplying the spring rate with the motion ratio squared (because the leverage increases both the force on the spring and reduces the compression). 79-93 Mustang lower arms have a motion ratio of 0.5, so with a 750 lb/in spring, you'll get a wheel ratio of 0.5*0.5*750= 188 lb/in. A vintage mustang has a motion ratio of 0.6, so a softer (~520 lb/in) would result in the same wheel rate. This also explains why the rear leafs are so much softer than the front coils. The leafs attach directly to the axle, so the move up and down with the wheel at the same rate, which makes the motion ratio 1.0. That means that the leafs can be about 3 times as soft as the front coils for the same wheel rate.
Actually, wheel rates don't mean so much either. If you really want to compare different cars, you need to look at the ride frequency, which also takes the unsprung weight into account. As I understand it, it's the frequency that more or less determines how comfortable the springs are. A nice article on this topic can be found here (springs and dampers part 1): Technical Papers - OptimumG
The original GT springs were around 290 lb/in I think. Boss 302 coils were about the stiffest the original springs got at around 350. A quick calculation shows that the ride frequency of both is very similar (1.2 vs 1.25 Hz), because the Boss has stiffer springs but is heavier too.
The frequency goes up with root of the spring rate, so installing a 600 lb/in instead of 290 lb/in GT springs doubles the wheel rate but ride frequency goes only up 40%. Enough to be objectionable, but not as much as you might expect by just looking at the spring rates.
Since it's largely a matter of preference, you would need to test them before you know what you like and since you already have the GT springs, I would install those and see how you like them. If you don't, switching to stiffer coils isn't that expensive or time consuming.