London again...

Going to London again tomorrow. Bringing back my I-Book to Regent Street. I am not happy with the PC2100 Ram they upgraded for me. The default speed for the I-Book is PC2700, how can they put a PC2100 for me? I made a call to them but they cannot help me, at last I said I will bring this I-Book back to their shop. At the same time I did e-mail them. They told me the RAM they upgraded is compatible. I knew it is compatible but the performance will be different.