Well I think it's an oversimplification to use flow as the only parameter. There are so many variables which interact, you could really wrap yourself around the axle. As stated, O2 availability is a key factor since these are aerobic bacteria. But there are also other requirements. Most people only consider Ammonia and Oxygen when thinking about these bacteria but in fact, nitrification takes a lot more than just these two chemicals. For example, some bacteria also consume organic Carbon. That's why adding peat and mulm to a filter bed or to the sediment helps bacteria. The bacteria that obtain their Carbon from organic sources such as peat are called heterotrophic. Some nitrifying bacteria are autotrophic, which means that they obtain their carbon from inorganic sources - like CO2. In order to produce energy, bacteria also need PO4, Magnesium and Iron.
In a planted tank these factors are significant, and we shouldn't forget that a "waste" product of photosynthesis is Oxygen. So if we are dumping all these components into the water it may easily be that the flow rate in the tank from that standpoint becomes less relevant simply because of the possibility that bacteria are being presented with an unlimited supply of all these components from their point of view. Therefore if I've saturated the water with O2/CO2 does it really matter to the bacteria whether I have high flow or low flow? They are still presented with an infinite Oxygen/CO2 rich environment. The microbe limiting components for a healthy, stable planted tank is more likely to be the Ammonia itself because healthy plants will compete with bacteria for this resource. If this is the case then what are we worried about? Limited Ammonia? That sounds like an awfully nice problem to have, in which case I don't have to try to figure out whether I need a combination of high flow and low flow, or which is better in this environment.
At some point, there is a minimum required flow, below which the bacteria are not producing enough nitrification. If the filter is clogged with detritus for example, this is a classic case where flow is blocked, which stifles Oxygen availability. The detritus breaks down and releases ammonia which goes unprocessed. When ejected into the water column this can lead to some forms of algae.
Since we really don't have measurement showing what flow rate through a filter, in what tank, produces what level of nitrification, it's a very tall order to compare the effectiveness of one filter rating versus another in a given tank. I can't find the link right now, but I recall an Ammonia pull-down test done by a discus club. The combatants were a 1700LPH Eheim 2180 versus a 2300LPH Fluval FX5. The Eheim smacked down the FX5 significantly, even with the lower rated flow. Why? Simple. The 2180 has a bigger bucket and therefore a much higher media capacity so in this particular case the lower flow rate was trumped by higher bacterial population. Does this imply that a lower flow rate is better than a higher flow rate through a filter? Hardly. That test was in a bare tank with kitchen ammonia being tossed in the tank. In a normal planted tank, would there be as significant a difference? Hard to say, because the conditions and the dynamics would be totally different.
If I have a choice in a tank to have multiple filters, I'm goin' for the two biggest, baddest filters possible, with as much flow rating as possible simply because I'm a flow/distribution lunatic. And that flow rate is gonna be more useful to me in terms of nutrient/CO2 distribution, and will solve a hell of a lot more problems than what I could measure in terms of nitrification.
Cheers,