The Radian has side impact protection (SIP) in the form of the EPS foam in the shell. It's not going to offer as much as a seat with true side impact protection (tsip) like the blvd, or even one with a deeper shell like the MA, but the presence of the foam means it has SIP.
There are no standards in North America for side impact testing, and in general, manufacturers who put in EPS or EPP foam consider their seats to have SIP because the EPS/EPP foam reduces the likelihood of injury from the child impacting their head on the seat shell. I know for sure that Britax does conduct crash tests using side impacts in addition to the frontal crash tests required by FMVSS and CMVSS 213 standards, I'm not certain if other manufacturers do or not.
A 5pt harness in and of itself offers a good deal of SIP, having EPS/EPP foam also adds a lot more SIP. A seat with a deeper shell is going to provide more of a physical barrier between an object and a child, but when talking SIP, we're really talking more about reducing the risk of injury from child contacting the car interior (in the case of booster vs. 5pt harness,) or the hard shell of the car seat (plastic shell vs. an EPS/EPP foam lined head area.)
To me, EPS/EPP foam is more important than the actual depth of the seat shell. I'd rather have a radian with EPS/EPP foam and the lower profile, than a seat with a deep shell but no EPS/EPP foam. Side impacts where severe cabin intrusion happens have a lower likelihood of occuring than just a side impact crash on it's own. EPS or EPP foam is going to provide increased protection in all types of crashes regardless of the speed of the crash. A deeper shell isn't really going to provide an increased degree of protection unless you're looking specifically from the point of preventing an object that has entered the passenger compartment from being able to contact the child in the car seat. In a crash of that magnitude, I'm not sure that a deeper seat shell in and of itself would provide that much more protection because you'd also be talking about high speeds and it raises the question that if an object has already entered the cabin space, is it likely to just go through the seat shell too?
I'm not sure... and it's pretty much speculative. What I do know is that the foam provides added protection, and while a seat with the foam and a deeper shell would likely provide a deeper level of protection in terms of a physical barrier in a severe side impact crash, I'm not sure how much of a difference injury wise there would be the vast majority of the time.
It's important to remember that carseats aren't designed with the intent of being the primary protection to a child, they're designed to keep a child in place in a crash & distribute the crash forces over the child's entire body, allowing the child to ride down the crash forces while the physical crash zones of the car absorb and reduce the force that is actually transferred to the occupants. A properly used car seat allows the vehicle itself to absorb as much energy as possible and then distributes the force over the child's body. The EPS/EPP foam is going to make a difference in that area because it'll protect the child if they hit their head on the seat shell, but as for how much difference a deeper shell would make, it's speculative... Ultimately, the vehicle's crash test ratings are what determine the true risk to occupants - the best car seat in the world won't provide enough protection if the vehicle you're transporting your kids in has awful crash test ratings... just something else to factor in to the mix.
