What you're planning sounds straightforward (could be done by knocking off the front trim piece, trimming the plywood with a circular saw or jig saw, then re-attaching the trim piece), and since it's DIY, no expense involved, but you are decreasing the depth of the seating overhang to less than 12" (if I'm understanding correctly). Overhang on front of cabinet should be 1" minimum, then add up to 3/4" for the decorative back panel of the cabinets, so m/l 25.75" subtracted from 36" is 10.25". NKBA recommends 15" depth at counter height for comfortable seating. NKBA guidelines, specifically #9

I'd rip those tops and plywood out and let the fabricators worry about the new underlayment. I'd skip new underlayment anyway. Fabricators try to use the cheapest roof sheathing possible and I don't like the dissimilar materials trying to get along. Polish the edges and lay the new tops, 2cm or 3cm, right on the cabinets.

Thanks for the responses. Technically the wood is 7/8” long. It’s 36-7/8”. I’m just going to trim an inch. Honestly we don’t even use the breakfast bar. I’d take it off completely but I think it would look bad and we use it when we have parties for food. We can put a lot on this peninsula. I’m not really worried about 11” vs 12” depth but I really do appreciate the consideration. The installers are going to put down new plywood. One more question. My peninsula is 96-1/2” long and the prefabricated peninsula is 98” Long on top but but when I measured its 96-1/4” underneath for the cabinet. Do you think the installers will be able to trim 1/4” off the lip on the underside of the slab? It’s only supposed to have 1-1/2” lip but it must be 1-3/4”. Seems to me like they can trim 1/4 off pretty easily. Not like it matters. Or would it be wiser to take a router and buzz off 1/4” of the end of the peninsula/cabinet just at the top edge? I would be worried that the cabinet would splinter. My gut says the installers will just buzz a little off the cantilever edge underneath right? The slab is plenty long. The lip is just a little bigger than it should be. Like I said they told me it was 1-1/2” so i figures it would be perfect 98 minus 1.5” but I measured it and it’s 1-3/4”. Any thoughts would be appreciated. Honestly again I’m getting a smoking deal. I had quotes as high as 11k for slabs/fab/install. I have 2 bathrooms with 2 sinks each. One bathroom with a. Single sink. Laundry room countertop. And my kitchen has a built in desk that I’m doing in granite. My master bath counter is 10’ long. It’s all going to match and be the same granite for $5,500 installed.

Abandon your underlayment, please.

The deal looks great....until it’s installed and isn’t anything like you expected. Hind sight is always 2020.

Sophie I know many people who have used this contractor and have even seen a couple projects first hand. It all checks out. You run the risk whenever you hire anyone.

It’s a large company. I paid for the material and I don’t have to pay for install until I am 100% happy after its done.