Jowy would have escalated his plan and taken Luca out right away. Luca in charge of two nations and trying to conquer a third would go against everything Joey was trying to accomplish.
I don't know if Luca would have been able to successfully take Toran, anyway. He did well in Jowston because he capitalized on its disunity and strife. It was a badly organized and poorly run state that he could easily divide (with Jowy's help). Toran is a centralized, united nation with a strong leader and a full standing army. It's also isolated geographically by the Banner Mountains and the Badlands. Attacking Toran would have not been part of Jowy's master plan, so Luca would not have had his full support in that endeavor, which would be a big blow. Jowy turned the tide in the Alliance Wars by taking both Muse and Greenhill, and he would definitely oppose Luca if he tried to expand beyond Jowston (even if it was behind the scenes).
Without Jowy's help, Highland would have had no generals but Yuber and Rowd (Solon was dead, Kiba/Klaus defected, Culgan/Seed would support Jowy, Sasarai was defeated, and Lucia would never have joined up). Luca's powerful, but I'm not sure he could take out the combined forces of Kwanda, Kasim, Sonya, Kai, Camille, and Valeria (along with the Dragon Knights) with just those two (especially since Rowd doesn't actually do anything in battle). Plus, I'm sure Leon would have joined up with Toran as well.