Well, if you assume that LFS is real life (it is, isn't it?), and that the exhaust was the only thing changed between s1 and S2 on the XRT, then the single exhaust weighs more (:S), but produces an extra 1hp at PEAK (might be less everywhere else). But it's likely that other tweaks were done too to produce the extra weight - maybe a different carpet density, or electric windows. Thus the lightening of the exhaust system was countered by more 'goodies' as standard. The additional power is unlikely to be a result of the exhaust change, but maybe a different cam, or a slightly different combustion chamber design etc.
Of course, if it IS just a computer game then I expect Scawen just tweaked the variables a bit, or some part of the system, at the same time that Eric decided a single exhaust looked better. But that would be a boring answer.