First, let's bound the discussion with fore-aft weight distribution relative to your original comment: "Race car drivers seem to be highly concerned with how a car balances thru braking, accelerating, and turning..."

Aside from the nose-bleed level concepts of weight distribution (50/50 is always the benchmark for good handling in performance machines) and basic cornering and braking concepts (friction and inertia are what they are), there are a few things regarding road-going motorcycles such as steering geometry and steering (to a certain extent) that can be transferred to bicycles, but that's about it for the motorsports connection. This is primarily because: (1) the operator to machine weight ratios are vastly different [bicycle ~9:1, motorcycle ~.3:1, sportscar ~.05:1], (2) CG is not limited to the horizontal and the vertical CG on a bicycle is significantly higher than on motorcycles or sportscars, and (3) the amount of power, how it is used, and how it can be delivered to the tires for different effect is obviously quite different, e.g., you can use the throttle and brakes to drift a motorcycle or car through a turn as well as to weight and unweight the front tires for different handling effects: a bicycle limits you to the use the brakes.

That said, as Elrey notes, bicycle and tandem rider weights are what they are. Moreover, because the rider's weight makes up the vast majority of a moving bicycle's mass, it also means that fore-aft distribution of weight is dynamic as riders can sit back, remain neutral, or shift their weight forward, as well as to the left or right of centerline while cornering and intertia introduces yet another variable. A bicycle rider can also exert downward torque on their bicycle's frame/wheels/tires via the handlebars, saddle and pedals to influence cornering. Now we need to factor in bicycle frame design, rider fitting, steering geometry, and of course the rider's ability to apply their knowledge of bicycle handling to the machine they're riding and the road conditions.

Do you see where this is headed...??? In the big scheme of things, fore-aft weight distribution on an upright, conventional bicycle is not something I'd spend a lot of time being concerned with.

Now, when it comes to tandems they are not, at least to the best of my knowledge, designed to compensate for significant fore-aft differences in rider weights beyond the basic need to increase torsional stiffness / reduce frame twisting. Therefore, as rider weights become significantly skewed to one end of the bike vs. another, handling will change... sometimes dramatically. For example, a tandem with an adult captain and a small child stoking will be far more prone to oversteer and/or a rear wheel wash-out in a fast, aggressive cornering situation. Reverse the situation and put a very lightweight rider up front as captain with a significantly heavier stoker in back and you could reasonably expect the tandem to understeer. As to how severe and or noticeable these conditions would actually be to the captain, that gets back into all of the other variables mentioned above.

OK, now lets look at weight differences associated with tandem handling qualities outside of braking, accelerating, and turning..."

It goes without saying, in most cases once you have a stoker of adequate weight to provide good rear wheel traction, any additional weight -- and the higher that weight is carried -- degrades the overall handling qualities of a tandem. However, it's not the fore-aft weight distribution that degrades your handling, per se, it is the exaggerated steering control needed to deal with the stoker's side-to-side weight shifts and body movements that come into play and that torque the tandem's frame which result in unintentional steering inputs. Captains with regular skilled stokers who are smaller and who have never had a larger "guest stoker" will not usually have an appreciation for how significant the impact on handling can be from a larger stoker, stoker's who move around a lot, or stokers who "stiffen-up" every time the tandem leans over for a turn. Likewise, captains who have larger stokers or stokers who are, shall we say, not all that well disciplined or who don't ride cleanly, will never appreciate how differently a tandem will handle with a smaller or more skilled stoker aboard.

To your last question regarding rear stoker compartment length, moving the stoker back will amplify any problems that a team has with highly skewed stoker weights or "stoker steering". Short tandems trade-off stoker comfort for better handling (period). Long tandems trade off handling for stoker comfort. The handling quality penalty for a longer tandem can be mitigated to a certain extent by builders who can figure out how to use materials or design frames that are more resistant to flexing or twisting, but that's about it.