In order to know whether the dish will act as a 23 inch dish or something larger, you need to know the characteristics of the LNBF - its "illumination pattern".
Manufacturers generally give just a single spec for this, the F/D ratio. But really what you want is a pattern of reception at a given distance. I would guess that the patterns are roughly circular, in which case your dish will act as roughly a 23 inch.. maybe slightly better because you will get some benefit of the radiation coming in off of the pattern.
So you really need to know the F/D of the dish, F/D of the LNBF and its illumination pattern to answer the question well.
Bottom line is, if it doesn't work, just buy a second dish
