I think it really depends on what you want out of your table. If it is going to be more of a recreational table for family and friends I would say there is no harm and no foul installing "incorrect" cushions. If you want your table for somewhat serious competition and to prepare you for tournaments I would say you really should spend the extra money to have cushions correctly fit to your table. If whoever installs the "incorrect" cushions can get the nose height correct and the angle of the cushion close so it has proper rebound it should be good enough, many people have tables like this and claim they play well. The playing area will shrink by 1/4" I think, not a big deal but the table will not be twice as long as it is wide anymore, I think that is a big deal and your diamond system will not play "on track". If you do not modify the rails you will also be left with large pocket openings, I have heard "shimmed" tables do not play very well. I had my GCII modified for current cushion profiles and had the rails extended for 4 1/2" pocket openings, I am very happy with the results, I think it plays better now than it did with the original spec cushions and rails. I also play league and many tournaments on Pro-Cut Diamond tables so I wanted a table at home with similar sized pockets.