Yup n=3 is the limit for IsStandard right now.
That isn't true. If you read the code, the limit is with the size of sigScript if you are using P2SH:
txin.scriptSig.size() > 500
Signatures are of length 72 bytes and public keys are of length 33 bytes (if compacted) so 4 of 6 is about the limit. I've managed 3 of 4 and it passed as a standard transaction.
This 500 byte limit is in IsStandardTx() check. You can still get your 10-of-20 multisig transaction included in block if your really need it.