Have a Hoover TT, and used the spin drier more than washer until purchased a separate unit. The older Laundry Alternative unit holds more than the Hoover (a full eleven pound load from the Miele or AEG Lavamat washers), and is a bit more quiet.
In terms of usefulness for thick and thirsty towels, hoodies and or anything else that really holds water, spin dryers can cut down time in tumble dryer considerably.
You are onto something in wishing to get a centrifugal extractor (spin dryer). In commercial laundries it has been a long held view that the best (and cheapest) way to get water out of fabrics is via mechanical means. Less water left after extraction means less energy used by dryers or ironers to "bake" that water out.
This being said if you are taking items spun already in your washing machine, the amount of "extra" water removed will depend upon several factors.
The largest is how fast your washing machine spins. Greatest gains in water removal come from going to 1000 rpms from say 800 rpms. More still between 1000rpms and 1200rpms. After 1300rpms the gains are increasingly small as not worth bothering in terms of energy savings.
Of course as you noted already having a spin dryer means you can have "high speed" extraction for long as you wish. My AEG will go to 1800rpms, but for only the last three or so minutes. In an effort to prolong lifespan however rarely set final spin speed above 1200rpms. If things need to be drier will bung them into the spin dryer for several minutes.
Here is my spin dryer and an older review:
https://www.tomsguide.com/us/green-laundry-purifier,review-1298-2.html
On a full load of towels and other thick items spun in the Miele at 900rpms, the spin dryer will easily remove one half to quart plus of water after four minutes. Same load done in the AEG and spun at 1200rpms will yield about a cup or more. These are all rough guesses going by how much water emptied out of catch basin.
Still has have always said, a full load of towels washed in the Miele, then spun in the spin dryer will try to a crisp in the little Whirlpool compact dyer in 45 minutes to an hour. If I take them to local launderette and use large gas commercial dryer things are done and toasty in about 20 to 30 minutes.