Zeh Keli V'Anveihu in the Cheftza or the Maaseh??

The first mishna in arvei pesachim 99b says that near mincha time a person shouldn't eat anything on erev pesach. The Rashbam explains that the idea behind this prohibition is that we want the matza that night to be eaten b'teavon - with appetite. The Rashbam adds that this is "mishum hiddur mitvah". I once heard someone point out the following interesting point on this Rashbam. Generally speaking, hiddur mitzvah (learnt from the pasuk of zeh keli v'anvehu) is reserved for cheftza shel mitzvah. For example, a person should have tallis naeh (a nice tallis), tefillin naeh (nice tefillin) etc. This is the way the din is brought in the gemara. Would we say that this din of hiddur mitzvah applies even to the maaseh mitzvah? Is there a din of hiddur mitzvah even in the performance of the mitzvah?

From this Rashbam it seems that, yes, the din of hiddur mitzvah applies even to the performance of a mitzvah. An even further chiddush you see from this Rashbam is that the chachamim have the power to make a gezairah that "forces" people to do hiddur mitzvah. This is certainly a strange idea that requires explanation. Anybody?