My little dude, was also born 8 weeks early and spent 7 weeks in the NICU. It's tough to guage these things, with preemies (and I think all kids) I was immediately told to throw expectations out the window. That everything I read and looked up on-line would only hinder and not help me, help my son.
When he was born I was refered to Glendale Memorial Hospitals Early Stages Infant Clinic and they evaluated his progress every three months. At first, I felt like he wasn't ever going to catch up. He had a hard time sitting up, rolling over and carrying himself in the early days...at their suggestions he started seeing a Occupational Therapist for an evaluation, and it was the best thing I ever did!! We went twice a week for almost six months, and then once a week for three months and he was done! It helped his walking progress faster and his motor skills catch up to his peers, and truly they gave me some insight and suggested activities I could do at home to help my son along. It was a bonding experience and amazing. Check with the NICU your son was in, and see if they have a sister program for these kinds of things. We didn't qualify for any of the free stuff, but there are usually programs around that can help. Or contact your insurance provider for names of clinics in the area.
As for the talking, my son is now 2 years old and has just now begun to string together sentences. It took him a while to catch up, but at the suggestion of the pediatrician and therapists, I bought all kinds of early reader books and spent tons of time reading to him and pointing to words and identifying objects by pointing at them. We'd take walks and I would talk to him and ask him questions. My son's doc says, he's doing great and that we shouldn't consider speech therapy just yet. He began with full words at about 16 months and just sort of stayed there for a while...not progressing past that until almost 20 months with phrases and word sets.
Using the adjusted age is better for developmental stages...that is what my son's doctor uses at Well Baby Checks and then shows me the typical results for a child his birth age. I always felt like using the birth age to check his range made me more consicious of what he WASNT' doing and not what he WAS doing...
Don't worry about how the charts and guidelines...unfortunately, there is not set meter for preemies and their development. I've looked so much up on-line and sometimes my son hits the mark and sometimes he doesn't. My son's doc said by age 3 most preemies are completely caught up, with growth and language skills and at that age is when we'll evaluate anymore therapy for speech or otherwise.
Good Luck and just remember, you're doing the best you can and so is your little One.