At the age of 24, Maul floored TPM Jinn in saber combat, who was said to be on par with TPM Windu in terms of swordsmanship - so I think that it's safe to say that TPM Maul > TPM Windu in saber combat.
Windu improved so much in those subsequent years (13) leading to ROTS, that he was powerful enough to be in the same league as Yoda (maybe slightly less powerful). This is clearly a huge improvement - from being on par with TPM Jinn to being in the same league as ROTS Yoda.
Now Maul was very young as of TPM (24) - he was still a sith apprentice and thus at an early stage of his development whereas Windu by TPM was already a senior member of the council and at the end of his jedi career, so seemingly on a relative scale wouldn't improve too much in those 13 years - especially considering as a senior member of the jedi council hardly saw too much combat whereas Maul as a sith apprentice (a tool to carry out the wishes) would likely see much more. Now I think it's safe to say that Maul would have improved more in those 13 years than Windu did.
These points lead me to believe that by ROTS, Maul would have gotten to Yoda's level (possibly surpass him), and he would have been 37 at this point. Now it seems that force users generally reach their peak by the age of 50, so add in 13 more years (Yoda would likely regress in these years - his saber capabilities is dependant on his force strength) and I think it's safe to say that Maul would pwn Yoda.
Sorry if this is not too clear, but I'm pretty sure that my reasoning is correct.
Oh, and this is only in regards to saber combat.