Well at least you didn't show here the Eheim price. You may not have made it to the door!
The 10X is the filter "rating" which filters never deliver on their best days assuming they are filled with media and are mounted at typical locations below the water level. They normally deliver only about half of their rating. The real number we had was 3X-5X the tank volume in actual pump delivery. Instead of complicating life with calculating real versus rated it's easier to go with 10X rated or nearest available. The added advantage of a high filter rating is that you also get a high filter capacity in terms of volume.
Some find the additional powerhead solution obnoxious looking so it's easier to have this flow throughput from the filter as opposed to mounting another device. I agree with sending the effluent across the short axis (back to front) which really helps carpet plants. I advocate the use of spray bars mounted on the back glass which lowers the exit velocity and spreads the flow more evenly across the long axis. Multiple spray bars can be ganged to reduce turbulence further.
The question though is what is enough and in what type of tank? A general rule of thumb ought to be good enough for most applications. 10X the filter rating
always works better than any head scratching associated with trying to adjust the recommendations for every conceivable tank configuration. Sometime you can get away with less or for one reason or another a 10X rating cannot be accommodated so just get as close as you can. Another issue to consider is that as the tank fills in with plant growth and with the inclusion of hardscape the flow distribution becomes blocked and stagnant areas can become more prevalent. Moreover, the added biomass of a few months growth will require more nutrient and CO2 delivery than was necessary due to higher biomass.
Certainly, smaller tanks, and lower lit tanks can get away with lower throughput. Non Co2 tanks in fact can get away with a lot less flow and filtration because things happen a lot slower in those configurations. But more flow is always better. Large tanks have a real problem with flow as there are huge stagnant areas due to energy dissipation of the effluent as it fills the space. In large highly lit tanks, flow is even more important. My tanks have always been on the large to very large side and I have empirical evidence that, all other things being as equal as can be, higher filter capacity and higher flow reduces the occurrence of algae or facilitates more rapid recuperation from an algal bloom.
I can safely guarantee you that if you did upgrade to an FX5, suitcase and settee notwithstanding, you would see a substantial improvement in water clarity, nutrient distribution and plant growth because you would be delivering nutrients/CO2 with greater efficiency, allowing your plants to produce greater levels of Oxygen, which would generate and support higher levels or aerobic bacteria - above and below the substrate - which in turn would accelerate nitrification. Of this there can be little doubt.
By the way, living on a brightly lit Hollywood stage in a glass box, being fed by hand several times a week in someones living room can hardly ever come close to being a natural habitat for any fish, but I do understand the issue of turbulence stressing this species.
Cheers,