While his operas may be symphonic, I sure wouldn't expect him to write better, more convincing works than Tchaikovsky. The one he did write is pretty godawful and demonstrates clearly that he was a dramatic composer not suited for the rigors of the symphony; and there's nothing wrong with that. The Russian did manage to write masterpieces in opera and symphony, while the German only mastered the former. Wagner was not at home writing purely instrumental music as the symphony and works like Polonia and Rule Britannia make all to obvious. His one success was Siegfried Idyll, but then it was derived from operatic material.Most would agree that Wagner's operas are uncommonly "symphonic". And I would also expect him to be able to write more convincing symphonies than e.g. Verdi or even Tchaikovsky...
Maybe they would have been great, but it's pure speculation with little basis. I don't know which other composer wrote the "most Wagnerian" symphonies... any candidates?
What is a Wagnerian symphony? There have been many composers whose music can be described as Wagnerian if we're looking at time scale, chromaticism, free-wheeling structure and intensity. Bruckner is always a candidate, but guys like Klughart, Rott, Wetz, Hausegger, and even Raff to some extent, fit the description. The work Nirwana by Hans von Bulow is really Wagnerian.