I have bilingual kids. DH speaks English to them, I speak another language to them. What you said about your 2nd language applies to ours as well.
From what I know bilingual children face a few disadvantages at the beginning but in the long run they can only gain from it. At the beginning a typical bilingual child has similar vocabulary size like his age mates. While the total number of words is similar, it is distributed between the two languages. Therefore the child may look like he knows fewer words than his age mates. This usually evens out by K.
In our case, DS7 started reading in English at the age of 3. He read his first few chapter books still way before turning 4. He learned how to read in his second language at the same time like reading English. Don't ask me why, it was his idea not mine
DS5 started reading in English at the age of 2, chapter books at 3. He is currently reading Harry Potter. Somewhere along the way he learned how to read in his 2nd language. We found out about it when he was about 3 1/2.
All that said both of the boys read almost exclusively in English these days. English is also their stronger language now but they do speak both languages each day.
As for testing, DS7 is DYS. When tested at the age of 5 his vocabulary was his relative weakness. His VIQ was still in 99% though. The psychologist even put in his report that his verbal scores were probably lower due to him being bilingual. She said that it should no longer be an issue at the age of 7 and up. If you ask me, his vocabulary is still his relative weakness and always will be. That's mostly because he is a mathy kid and his math aptitude will always trump his vocabulary abilities.
DS5 on the other hand scored 19 (the highest score) on vocabulary. I was shocked when I found out. According to the same psychologist, vocabulary is his significant relative strength even though he scored bunch of 18s on different subjects and his PIQ is greater than VIQ.
DS5 has always been more verbal than DS7. Would they both scored better if they were not bilingual? May be, but I would bet my money that their real VIQ isn't significantly higher than what the test showed.
HTH
