I bought a pair of Sidis after wearing Shimano shoes for a long time. I didn't like them & sold them after about 4 months of riding them. I only bought them because Shimano discontinued my favorite model. I didn't think they were any kind of improvement over the Shimanos.

That said, maybe there's some kind of insert you can use in the heel area to snug it up. If it were me I'd take a look at the local drugstore for Dr. Scholls or Spenco displays. You might also take a look here: Women's Shoes, Men's Shoes, Walking Shoes, Sandals, Slippers, Socks, Support Hose, Foot Care, Orthopedic & Comfort Products :: FootSmart

 

I may end up selling these too and going back to my old Shimanos, which only have straps (which I find much easier to use anyway). Do much for trying something new.

 

I couldn't agree more assuming the Shimanos fit you well. My Specialized Pros have a buckle & I'll be damned if I can tell any improvement over straps. A you ride your feet swell & for me I've found there's no need for adjustment. Almost all cycling shoes have very little if any leather in them. They don't stretch.

 

I wouldn't give up on the Sidi's so quickly, especially for shoes without a ratcheting buckle. I don't have any references to cite but my years of experience have found Sidi road and MtB shoes superior.

I assume you know you can reach down and apply more tension on the buckle after 10-20 min of riding? Your socks compress, the shoes stretch, and a little tension helps snug them up.

There are also narrow and wide sizes in Sidi. You might benefit from the narrow, but I think the regular tends to run narrow.

 

Shoes - get the ones that fit. It appears that Sidi shoes do not fit. Get a different brand of shoes that do fit. Hope this helps.

 

Shoes are almost as bad as saddles...

I went: Vittoria, Sidi (Genius) and now S-works. Vittoria's had a flexy first-gen injected sole and never had a good heel fit. Eventually the sole and the upper separated here and that is what leads me to suspect the flexing sole-loose heel fit hypothesis.

The Sidi's started with great heel hold but lately I found that no matter how hard I reefed down on the straps and buckle my foot seemed to be squirming inside the shoe, both at the front and the heel.

The S-works (carbon sole) actually seems to have a looser fit for the upper but it feels much more secure. Perhaps the boa straps help a bit but I really think the stiffness has some impact...

 

My Pro 5's do not seem to run small. I tried a pair of 6.6's and they did seem to run small (and narrow) and I went back the the 5's. I do not have any slippage issues with the heel.