I'd definitley have a conversation with your NetApp Technical account team and see what they recommend. A few things to consider:
I believe IOMeter tends to read/write randomized data, so controller caching is less effective. THis means the 8ms response time you are seeing during your IOMeter tests may be a worse case than what you will see in production.
I'd ask about Flash Cache if read performance is a large issue for you. Flash Cache added to your controllers will give you a second, very fast layer of read only cache. It should improve overall read performance in most cases.
Less than 1 ms?!? SSD... maybe. Either pure SSD aggregates (Expensive!!) or Flash Pool aggregates with SSD on the front end and SAS on the back end. This is very high end stuff.
FAS 3250s aren't slow machines! They have lots of processing power. The larger units have more memory and expansion, and perhaps more processing power, but not much. You won't gain performance from a larger head unless you are stressing the 3250 performance-wise in the first place.
Hope that helps!