Obviously, yes - but a 550 MHz ar71xx/ ath79 AR9344 using the newer mips 74Kc core is with the back to the wall dealing with 100/40 MBit/s, yes it can do it, but right at its limits. 2*500 MHz lantiq VRX268 based on mips 34Kc can only cope with that speed using flow-offloading, it's very unlikely that first generation mips32 BCM6328 at 320 MHz can deal with that - and why would it, ADSL2+ is specified at up to 20 MBit/s (and I don't think any commercial implementation was using more than 16 MBit/s), so why would the manufacturer make it any faster than that?
Until pretty recently, ~50 MBit/s VDSL connections were considered top-end, with most users being at -and significantly below- 16 MBit/s, there was no reason for manufacturers to produce faster and more expensive devices for the mass market. This is increasingly an issue for users with older hardware, whose internet connection was suddenly (and finally) upgraded to VDSL2, cable or fibre.
To deal with 100/40 MBit/s comfortably, you need at least ~700 MHz ar71xx/ ath79, but better go a little faster and pick ipq40xx instead; lantiq VRX268 might be another attractive (cheap) alternative, which is hard at the limit at those speeds (but can do it using flow-offloading) but already includes a decent VDSL2+vectoring modem. See Top ten routers currently in use? - #4 by slh for suggestions, unless you can get a modern top-end ar71xx/ ath79 device for less than 30 EUR or want lantiq/ an integrated modem, I'd strongly suggest to go with ipq40xx to retain some margin for the future.