To get this out of the way ahead: I consider "15mins of slack" before a meeting not enough. When I come to work, I have to read mail and look at what I have done yesterday in order to know what it was, so I can tell the others. Others will need to have had a cup coffee before being able to start thinking straight. Where I worked we usually had at least 30mins, sometimes even more.
With these preliminaries out of the way: I guess at the very heart your problem is simply that you are mixing two different issues:
- When are employees expected to be at work and how hard this rule is enforced?
- When should your meeting be?
If the meeting is the only reason you keep pestering your devs for about being late, they will simply start to consider the meeting an annoying task causing them lots of trouble - something you probably wouldn't want.
If you have a hard rule for your devs to be at work by 8:30am, enforce that with all your might, never mind the meeting. Once you've done that, the meeting shouldn't be a problem anymore. If, OTOH, you wouldn't mind your devs to have a bit of flexibility, discuss the issue with them in order to settle on a latest time to arrive for everybody, add some "slack" to grab a coffee, read mail, and remember yesterday, and schedule the meeting at the appropriate time after that.
If people still come to work too late even after that, try to find out why. Are they simply unable to adhere to even a flexible schedule? Do they not care? Are they fed up because the company treats them badly in some other respect? Do they have serious problems preventing them from being on time every day? How to deal with this depends on what's the answers to these questions. (For example, if they have trouble commuting, maybe the company could do something to encourage them to move? OTOH, if they simply don't care, maybe you wouldn't want them in your team anyway?)
If you want an opinion from me as to whether to enforce a strict starting time or whether to allow flexibility: Would you expect your devs to leave at the same time every day, abandoning their work at the exact minute, regardless of what they are doing? I seriously doubt that. However, if a company expects them to finish whatever they are doing in the evening before they leave for home, why would a company expect them to not to do this at home, too, before they leave for work?
My experience with a daily standup meeting from when I worked in an XP team: We had flexible hours, with 10am the latest time one should arrive (if you came later, you were expected to either have announced it the day before or, if something happened in the morning, call at work and give a reasonable excuse), and the meeting was held simply when everybody had arrived and had time to settle in, usually before 11am.