My first son started speaking at 5 months and by a year was able to have full conversations. By 20 months he could direct me to the shops and taught himself to read by 3. he also always seemed to know early maths skills. He sat early (5 months) but crawled and walked on average times (10 months and 14 months). We pegged him at about level 4 on Ruf's scale.
second son's first word was at 3 months and then nothing for ages so he took longer to speak clearly but had an advanced vocabulary from when he started speaking. He also taught himself to read before he was 3 and now at just over 5 years is a voracious reader. his maths skills are also astounding and he grasps things faster than older kid. Again, physical development was almost identical to first son, except that he is really strong at ball skill sports. We peg him at level 5.
third son's first word was also at 3 months, and although everyone says he speaks well for his age it's not the same as the other two, and sometimes I am unsure about him. Until he does things like randomly spelling out words with foam letters or funny things like that. I don't know his approx level just yet - my Ruf book is missing *fume*.
We are now a more ecclectic homeschooling/unschooling family so there is a lot less that we formally do with the kids. Having said that, he follows a lot of the same "signs" that oldest boy followed so for now I am assuming same LOG - his drives and intensities are the same except for being MORE tactile sensitive (bummer).
For me, the level is a good indicator of the speed at which they will need to work more than anything else. But honestly if you are already having doubts, chances are school without modifications will be too slow and cumbersome.
I loved Ruf's LOG guide mostly for the anecdotal stuff - it helped me to understand that "normal" for my kids is different, and that's okay. It also helped me come to terms with what I could expect more or less and it helped me to settle down a lot to understanding and accepting my kids just the way they are.
