As long as I've read Bluetooth 4.1 Core Specification, Bluetooth LE pairing (key exchange) is done with pseudo-random and hash. In other words, it is not based on RSA public-key key exchange. There is notion that says future version of Bluetooth will include "elliptic curve cryptography and Diffie-Hellman public key exchanges" (Volume 3, Part H, Section 2.3.1), but still no mention to RSA public key exchange.
RSA is well-defined, widely adopted, relatively lightweight (compare to DH or EC), considered to be very secure public key algorithm. Security on internet (SSL/TLS) is almost dominated by RSA (though TLS also supports DH, rarely used in practice). RSA is also popular in WiFi enterprise security, which uses essentially same TLS protocol over EAP framework.
Definately 4Kbit hardware-accelarated RSA engine is something "nice to have". However, as long as I read Bluetooth core spec, RSA is not adopted as standard as I wrote above.
In the end my question is, is RSA-based Bluetooth security is propriately to Broadcomm chipset, or are they (Broadcomm) want it to be part of Bluetooth SIG standard?