Why would you use iBGP over eBGP. i.e why use a single AS with an RR over multiple AS's using eBGP. Is it simply for peering design simplification i.e large amount of devices all peering to a set of RRs?
When using iBGP, you the router must have a full mesh, or you must use a mitigation. Route reflectors are one mitigation. There is another mitigation that is similar to what you suggest, and it is called confederations. It basically breaks your AS in sub-ASes and uses eBGP between the sub-ASes.
You can search for BGP confederations, and you will find documents, such as IP Routing: BGP Configuration Guide:
BGP Routing Domain Confederation
One way to reduce the internal BGP (iBGP) mesh is to divide an autonomous system into multiple subautonomous systems and group them into a single confederation. To the outside world, the confederation looks like a single autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Even though the peers in different autonomous systems have external BGP (eBGP) sessions, they exchange routing information as if they were iBGP peers. Specifically, the next hop, Multi Exit Discriminator (MED) attribute, and local preference information are preserved. This feature allows the you to retain a single Interior Gateway Protocol (IGP) for all of the autonomous systems.
To configure a BGP confederation, you must specify a confederation identifier. To the outside world, the group of autonomous systems will look like a single autonomous system with the confederation identifier as the autonomous system number.
Route reflectors are simpler to configure, and it is easy to screw up a confederation design and configurations. If you have a large AS, buying many AS numbers can be problematic, and confederations can internally use private AS numbers because you do not advertise them outside your AS. Some people like router reflectors, and some people like confederations. Having multiple, real ASes really isn't practical.