Let's see...
If you set the step rate to 100 emails, then you'll be sitting there clicking "next" buttons for 130 cycles. Given maybe 1sec/email you'll be sitting there for 13000 seconds or over 3 hours.... And at the same time hoping you're complying properly with the industry spam requirements and that your server is setup properly to send batch emails in that quantity without being dropped by the big providers. Not sure what your customer's time is worth, but clicking next for 3 hours might get a tad tedious....
That's not how it works any more. While it is browser based, it auto-refreshes all on its own at whatever time interval CS-Cart devs assigned to it. Once you hit "save and send" it goes until it's done without any other user intervention. The number of emails "per step" that is configurable in the newsletter addon simply determines how many emails it fetches to process per "refresh" (step). It's a "set it and forget it" situation except you have to leave your browser open for it to cycle through the entire subscriber list you assigned to the newsletter you're sending. Even still, I can imagine this would get extremely out of hand for a store with subscribers in the 100s of thousands, but even with 13,000 subs in this situation, it's a relatively painless and quick process. My actual concern is I feel it's too quick, and often triggers delivery throttling with major email providers' servers, which is why I'm working on modifying it (slowing it down) in this case. It would be lovely if this was a configurable setting through admin rather than having to go code diving, but such is life. 
You absolutely have to have properly working reverse DNS and TXT/SPF records at the very minimum, but more focused DMARC, DKIM and the like really should be done properly as well for best practices. This often is beyond the scope of the more casual CS-Cart store owner, so I understand your concerns with that. I share them too.
I may eventually push them to Amazon SES. It's extremely cheap, no monthly fees... just pay for what you use.
Take Care!