I would go ahead. The callback method used is the 'older' traditional method payment gateways handled web-based transactions and hasn't changed at all in some time..
My expertise is with PSIGate and SecPay but you might check if WorldPay has a newer SOAP or RPC interface which (amongst other features) makes it unnecessary to send customer to gateway site and have to handle callback.
mitd-Made in the Dark
I've always been astonished by the absurd turns
rivers have to make to flow under every bridge.