ASIC gate count estimation and SRAM vs flip-flops
Building half gates from a NAND gate considering its transistor count is not always correct, from the next picture [Boylestad, Nashelsky (1992) Electronic devices &amp; applications] you can see that there is only one transistor and four diodes. Thus I bet that the digital design of logic elements is more extensive (consider also FET's which may have different configurations).

Similarly with flip flops, muxes and so much other devices. Indeed nand gates are the building block of digital electronics, because from them you can bassically design every other device, but the design normally changes and may be simplified or enlarged depending on the application, wheather it is for power, communications, data storage, etc.
Finally, as you said a RAM can be build form D FF's, but once again, the main difference between building one by yourself and usign an Integrated Circuit, is that the IC has been previously designed for an application in frecuency, power, simplicity, price, etc. whereas the one you build by yourself will be more personal and you will have to do the math on its performance, complexity and bulkiness.
I hope this has been helpful. Greetings.