If that is the case then you probably don't want to go with softer rear springs. there's a difference between the weight transferring to the rear and the body moving back (vs the wheels at least) It is no doubt a balancing act, but softening the rear too much (I'd speculate that the factory springs are softer than ideal, since they are specced for ride comfort etc as well, over-rider/overload leaves ignored for the moment).

It's a hard one to put into words (and I didn't do mechanical engineering, so I don't know the right terms, or even the specific math involved, but I can visualise and understand the various competing forces in my head clearly enough) but if the rear is soft it can delay the rear wheel downforce, instead of cushioning it and allowing it to maximise (obviously it is a balancing act between the two - too sloppy and no grip, to harsh, and it can shock and lose traction).;

Based on the current 60ft times (and to be fair they aren't exactly 'dreadful' and this isn't a 600bhp ultralight car on slicks) i'd actually suggest it is evidence that raising the front spring pivot point is going to help things out..

Essentially you have to come up with ways to increase the rear grip (which more or less means more downforce on the tyres) without it also adding weight to the car in proportion to the increase in downforce. In other words, if you increased grip by 10%, but did it in a way that there is now 10% more mass to accelerate there'll be no improvement in launch, and overall it'll now have worse power:weight and run slower again.

One trick therefore is to add weight to the rear, but to add it as far back as you pssibly can. Believe it or not I got this idea (*it makes perfect sense and no doubt others would have come up with it too) from an article about early super stock racing in teh US. They basically ran production/showroom spec cars with narrow mods allowed. At the time, winning races = big sales so the factorys got behind various racers to help. Winning meant everything, and obviously winning cars were heavily scrutinised, so they couldn't cheat - but they did anyway. What they did was to also secretly sponsor other cars run by privateers, but build them illegally, bigger motor, etc etc, and this faster than legal car would hopefully knock off the main competitions in the elinimations/semi finals, then they'd deliberately red-light or otherwise lose, so the cheat car never got scrutinised, and the official factory backed cars went through to the finals without as much competition.

Anyway - what they did among many other things was to custom make new rear bumpers that were full of lead (couldn't be seen from the outside, and never got stripped down after) or other weight (obviously well secured/integrated) Being right at the rear tip of the car, every kg of extra weight had considerable leverage, so it translated into 5-10 times that amount of extra downforce on the rear tyres. Nice gain, when you think about it that much extra grip for only a few kg extra weight (I forget how much they allegedly ran, something like 20+kg.

Obviously this would make for less optimal handling, as it puts weight so far back from teh centre of gravity and rear axle centreline that it would promote oversteer if cornering near the limit. But you could always just run the weight (strategeically placed, and using a spare bumper might even be the cheapest way) naturally none of the dattos has the sort of rear 'overhang' as US musclecars from that era had, so the potential gains/improvements are lessened, but hey, why not. I suppose other stuff like a relocated battery. could be tried too. Perhaps even the totally dodgy idea - like getting a 'stuffed' spare wheel and tyre, and filling it with water (and a little bit of air just to keep the valve from seeping any water) and fitting it in the rear as if it were a normal spare. dunno if that would be far enough to totally outweigh the costs of extra mass to accelerate. Might make for an interesting experiment.