Go with the turn over rule. Don't believe the manufacturer's "this filter is rated for X gallon tank size".
Usually it's recommended to have a x5-x7 turn over rate. I prefer to recommend at least a x7 turn over rate when going by the listed specs GPH.
The listed GPH is usually rated when the pump operating at level headheight and no obstructions. So when you factor in head height, obstructions from media, etc. the actual/realistic GPH is much lower than the rated GPH when situated for our typical aquarium use. This is more of a factor when dealing with filter pumps that operate with headheight, such as canisters or sumps that are below the tank since they have to pump water up higher. Filters/pumps that operate at level headheight, such HOB's, interal submersible filters, etc., just suck in and push out water, no need to really lift any water, so their actual GPH is closer to the rated, but there still is some GPH reducing factors such as obstructions from the media we use.
From what I have read, the difference in rated GPH depends on the brand/filter. For instance, SunSun canisters actual GPH may be closer to half the rated GPH, where as many Eheim's and Fluval's actual GPH is closer to their rated GPH ("GPH holds more true", hear the reviews of them being powerful outflows even though they have lower rated GPH than SunSun's). That's just from what I have heard, I haven't tested myself, but again, there are many factors that affect GPH, and we aren't sure if reviewers have factored those in (I give them the benefit of the doubt though. maybe Eheim and Fluval rate their filters in more realistic settings?).
I'm a fan of overfiltration, so the more the merrier as long as the current within the tank isn't very chaotic for the tank inhabitants.
The higher turn over rate, the faster/more times any wastes get passed through the filter, keeping wastes addressed sooner, which is a plus. The more GPH does help with circulation as well.
Those are just a preference though. I am not familiar with Eheim filters and their GPH ratings, but I am sure plenty of people use this or that canister filter that only equates to x4 or less turn over rate and have no issues. Bioload does play a factor though. Look at reviews or post another thread (with a title like "which Eheim filter for 55 gallon tank") or look up already asked and answered threads regarding which Eheim canister for a 55 gallon tank. By the way, I've heard there are rebranded Eheim canister filters (Ecco series?) under the Nat Geo brand (sold at PetCo/Petsmart), so they should be a good bit cheaper.
With all that said, I have used a SunSun 304B (rated 525 GPH, but actual is significantly less, closer to half that maybe) on a 55 gallon tank and it can do the job. It does lack a bit in outlet flow, and so you might desire to add a additional circulation pump/powerhead in the tank to circulate within the tank more. But yeah, just say actual GPH is 250 GPH and it can handle biological filtration perfectly fine (and my tank is overstocked). I haven't used Eheim/Fluval canisters so I can't comment on their actual GPH, so it's tough for me to say which filter/rated GPH of those brand canisters would work, which is why I recommend asking others who do/have used them, or look up already given reviews/experiences.
