Tiers only seem to be an issue in applying for Dermatology apart from that it's just research opportunities and opportunities to pursue masters or a PHD concurrently with a medical degree.I've never heard of tiers for med programs and certainly wouldn't let it be a deal-breaker in picking a university. I'd prefer to do 2 years as an unaccredited reg (maybe doing a masters, getting more references or doing research) than spend an extra 2 years in uni to do a research project which may or may not be published.
EDIT: Also, a lot of colleges stipulate that your research needs to be published within 5 years of applying for that college, so it should only really be a consideration in your last 2 years or so of your med degree.
If you feel like spending 2-3 years as unaccredited reg that may be fine but it won't help work/life balance. I'm just suggesting if you want to fast track then doing things whilst in medical school may help. Also don't forget you will most likely be marrying, having kids etc. so more responsibilities equates to less time so quite a few people suggest doing these things as early as possible meaning when you decide which specialty you want to pursue.
Research, presentations etc. should be within 10 years I believe some colleges accept up to 15 years.