Edit: wrote a wall of text, sorry!

Two drill-size motors, or 550 motors, can give powerful performance in the right robots. Cobalt is an example, it used two 550 motors on Banebots 16:1 gearboxes, 73mm Banebots wheels, the result was fast and punchy, despite being essentially two drill motors. I think it's safe to put the effectiveness of that machine down to good grip, good driving, and very good design/weight distrib.

However it's hard to get that sort of performance reliably from just two 550s without quite big bucks.

An option is to use 4x 550 motors, like we do in Tormenta 2. It's not in the same league as the likes of Satanix (S900s), but it does the job of shoving most machines around. Reliability is a major issue, but that might well be down to heavy handed driving as much as anything.

775 size motors are kind of halfway between the two. These are also sold attached to Banebots gearboxes, but they're not exactly cheap and only available in the US. They are also sold with gearboxes in "Magnum 775s", also US-only, similar prices to Banebots, and only a 20:1 reduction option. A custom reduction solution is possible but I think most 775-size are high rpm, 20,000 ish, so you're looking at complicated gearboxes there. That's the beauty of S900s, they're very low revving, so single-stage gearboxes are possible - reliable and simple.

There are several S900 clone-ey type motors on Ebay and across the web, as well as similar things like "Torpedo 800s/850s" - I'd love to get a pair of these and experiment but haven't had the opportunity.

A very powerful option, which would exceed most if not all feathers in the UK right now, is using Dewalts. The individual motors can be had on Ebay for not too much, UK based too, but without gearboxes. Once you add Dewalt gearboxes to these the prices rocket. They can be bought with gearboxes and custom mounts, so they are as simple to implement as Gimsons, here. Again, pricey and US based.

There is a motor size between 775s and 900s, of which I have four, called Johnson 885s. These are a little hard to come by now, but they are sometimes available here. At 18v, they do about 10,000rpm under load and can produce up to 600w. Chances are two of these will be going into whatever I/we build next feather-wise, so I will share results if that happens. On that same link there is a variety of Johnson motors in the 700 size range, you may well find something suitable there.

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In short, it's hard to make 550 motors be the basis for a uber-pusher without high cost, both in a money and reliability sense. Most other options with bigger motors that include gearboxes are expensive. There are several options for just the motors themselves that are comparable to S900s, but they will require custom gearboxes.

Hope that helps give some food for thought. I have probably forgotten an entire branch of the options if not a whole tree, sorry if I did!