Hmmmm, few things at work here.
The touring shell is the best aerodynamically of all the normal shells, this comes from an effect of the rear end that was replicated with the e30m3 shell, they share a rear end design known as the Kamm back.
This is a design where the rear end is truncated at roughly 50% of the rear profile area and is know to shed air in a similar manner to a vehicle with a fully tapered "teardrop" rear end.
Arguably the slope of the touring tailgate is better at producing this effect than the m3 stepped rear end/boot extension, however i think the m3 shell makes up for it else where.
Whit this in mind a diffused rear undertray could be made to really work on a touring due to the velocity of the air over the rear end.
The effects of this can be felt on the road with the touring suffering less overall lift and getting a couple of mpg more in the real world at motorway speeds.
Another thing that may play into your hands is weight distribution. A rear biased weight distribution is actually desirable on a racing car, this may sound backwards but its true and most notable in single seaters where there may be around 40/60 front/rear axle loads.
A rear bias achieves two things, firstly the loading of the driven wheels is better resulting in more traction in all conditions.
Secondly the breaking is significantly improved by being able to run about 50/50 bias (for around a 40/60 loading) as there is already more weight over the rear and as the load is transferred foreward the axle loads actually even up.
Combined result is better drive out of corners and later braking adding up to significantly reduced lap times.
Another benefit of the rearward weigh bias is the overall handling balance, however this is subjective and car specific.
It can result in initial understeer on turn in due to a light front end and very low front tyre loadings but i doubt an e30 could get enough rear bias to find this, a good example of this is how badly a touring will understeer when you stick several patio slabs in the back, can be a bit interesting.
Downsides to a rearward balance is that once inertia takes over that heavy rear end will swing out like a pendulum, think old 911, the touring does exhibit this to a certain extent in the way it keeps in line in a corner better than the saloon but swings out like a pig at the limit with less time to catch it.
Rear bias cars are much harder to drive than neutral or front bias ones and everything mentioned above only applies to rwd cars, fwd is another kettle of fish alltogether.
I think one of the biggest issues with driving e30s fast is actually the 50/50 balance, it leads to a situation where both power oversteer and lift off oversteer can be generated if ham fisted or unlucky and the front tends to want to push a lot when driving neutrally due to the way the e30 is set up (easy to feel when just tooling about and its slippery on a roundabout).
The ability of the car to be pushed into most handling situations leads to a car that requires a bit of care, where a more compromised balance would always tend to one direction and a driving style that avoids that situation can be developed the e30 could go either way and can always be in question.
I do think a less balanced car would actually be faster in most situations.
milliken and milliken's race car dynamics is the holy bible on this sort of thing if you want to dig in further and remember station wagons make better drag cars
A Touring with a DTM carbon airbox bark is appealing...! 
