The way I look at it, I want the guy on the other end of the phone at the converter shop to be on the same page as I am.
If that isnt the case, it's not going to be what you want.

One of my customers had a TCI 9" that had been redone by Lupo when he was still in Ct.
Worked really well for years. Finally broke it, and pretty much wasted it. Plus the technology had changed.
He ended up buying a bolt together deal($1400) from a pretty well known shop that was just way off.
Sent it back many times. They never even got it close. Speed way off, lots of slip, no 60', couldn't get it to cut a light(glide with a brake).
Interestingly, at the same time he bought his, his brother also bought one from them..... And that one worked well first try.
In the end, the shop basically admitted they were stymied by his combo and took it back.

He replaced it with a Hughes converter, which also has a fair amount of high gear slip(and low mph), but the 60' times were back to what they were with the old TCI/Lupo piece, and the consistency on the tree greatly improved.

When he finally stepped up to a bigger motor he got a new converter from PTC, and thàt one was pretty good on the first try.

Years ago when my friend was running an auto in his GTO in Stock, he had multiple converters from a few different shops, and each one went back at least once.
We never got any of them to do what we asked for, and felt there was a fair amount of performance being left on the table, both in ET and speed.
He got tired of spending $$$ and not getting any result for it, so he put a stick in it.
Picked up over .3 and 3mph on the first outing.