Ben,
The typical rule of thumb has been no more than a 3mm spacer with the stock hub hardware. If you use a spacer of 5mm or more it is advised for you to get longer lug studs or lug bolts to make up for the difference. Some people swear that 5mm is safe on stock hardware, but it's not worth chancing in the end.
If you're looking for a spacer greater than even 15mm, get hub spacers which are hub and wheel centric to avoid vibration. Hub Adapters/Spacers are extremely strong, and many companies have been running adapters on their track cars without problems.
I'm curious, what's the initial offset on the wheels now, and what ETs are you looking for in the final offset?
Also, by adding spacers, you're decreasing the offset