Government data indicates that the average six-year-old boy stands approximately 120cm tall. The average torso-to-leg ratio for adults is about 4:3; it increases from birth to maturity. Let us therefore presume that young Master Spacehunter's legs were about 40cm long.
Let us further assume that the target, the surface of the water in the toilet bowl, is suspended by the base of the toilet and depth of the water, at an elevation of 20cm. I obtained this figure by measuring the elevation of the surface of the water in my own toilet. It is, I believe, a reasonable estimate of the elevation of the surface of the water in most household toilets. (However, Spacehunter was raised by abos, so he may have been accustomed as lad to relieving himself in the bush. We will set this possibility aside for the purpose of our direct comparison.)
Let us also assume his penis, plus the cartilage that attaches it to the base of his torso, was initially 5cm long, which I believe is reasonable for a boy of six. And let us also assume he stood about 25cm from the target area.
The aperture of young Spacehunter's penis, therefore, had a vertical elevation of 15cm over the surface of the water, and a horizontal differentiation of 25cm. Simple application of the Pythagorean Theorem demonstrates that young Spacehunter's golden piss travelled approximately 29cm to reach the bowl.
The actual distance travelled by the piss itself would have been slightly longer due to the hyperbolic trajectory, but I do not possess the knowledge of calculus and physics necessary to model this effectively. I believe Dooj could help us here.
Anyway, let us assume that Spacehunter is now a decent 200cm tall and has a vigorous 16cm manhood. (If this estimate is incorrect, please, by all means, set me straight) Assuming a normal 4:3 adult torso/leg ratio, and thus a leg length of 114cm, the aperture of Spacehunter's penis is now elevated 78cm over the water, and maintains the same 25cm horizontal differentiation. (We must note here that the vertical trajectory now exceeds the horizontal). In any event, again applying the Pythagorean Theorem, we conclude that Spacehunter's piss now travels a nominal 82cm to reach the target.
Therefore, Spacehunter's nominal pissing performance as an adult has increased by as much as 180% above his performance as a lad.
If he is stocky, or well-endowed, then he would register substantially less gains. Other possible influences include African Muslim-style circumcision (which is administered during childhood and not infancy) or enlarged penis lips due to infection caused by his frequent sexual intercourse with sheep, goats, spacegoats, elves, abos and drunken, pasty-faced Kiwi wenches.
However, Spacehunter has since taken up WoW, meaning he uses the banjo less frequently but in greater volume, skewing any direct comparison. More research is needed to determine the effect of WoW-related urine retention on both the potential energy of urine emission as well as the urinator.
That said, I think that a 180% increase in performance due to changes in bodily performance far outweighs any probable modification due to increased skill, practice, or the like.
Are my figures and reasoning satisfactory? Spacehunter?