What is the s/mime encryption strength on iOS?
SHA-1 is most likely it. Here's what I've found to support that claim:
On the Cryptographic Message Syntax Services Reference document, it is stated that the S/MIME version used is 3.1, which is defined here and where it's stated that it supports SHA-1, SHA-256, SHA-384, and SHA-512 but:

The SHA-256, SHA-384, and SHA-512 algorithms [FIPS180-2] are not currently recommended in S/MIME, and are included here for completeness.

Even more, on the Certificate, Key, and Trust Services Reference document (although it's the Mac version), it's stated that the default algorithm used is the SHA-1.
In another document, it is said:

The most common hash function you will use is SHA-1, an algorithm developed and published by the U.S. Government that produces a 160-bit hash value from any data up to 2**64 bits in length. There are also a number of more exotic algorithms such as SHA-2, elliptic-curve-based algorithms, and so on.
For compatibility with existing systems and infrastructure, you may occasionally need to use older algorithms such as MD5, but they are not recommended for use in new designs because of known weaknesses.

So, although I couldn't find any direct statement confirming the use of the SHA-1 algorithm, clues point in that direction.