Electronic – Is the footprint or ball/land pattern for DDR1/2/3/4 IC’s a standard

If you're interested in solderability and manufacturability, then you should follow the recommended pad layout. I've never had a manufacturer's recommended pad layout give me grief, although I have run across a few vendors whose dimensioning requires a fair bit of headscratching and pencil-and-paper chicken-scratching in order to figure out offsets and spacing.

You are making some bad assumptions in your calculations though. No, pins are not often centered on the pads "lengthwise" but they are often centered widthwise. Y1 and C1 would most certainly be given by the manufacturer. The recommended land pattern will (in my experience) give more space for the pin on the "outside" of the pad and less underneath it. My guess is that that gives a good shape to the solder connection. You won't have anything in terms of heat dissipation unless there are a lot of grounds or you have a ground pad underneath. In the case of a lot of ground pins, you'll want to give them a lot of copper fairly quickly, but you'll want to connect the pad to the copper with thermals or you'll have soldering problems.

I wouldn't worry about minimizing board space, especially if you're not building a million of these. The half a millimeter you might save by shaving Y1 a little isn't worth it.

The current standard is IPC-7351B, which replaced IPC-7351A, IPC-7351 and IPC-SM-782 (in that order). Mentor Graphics have a free PCB land pattern viewer for Windows (old link; they rebranded to PADS) for all of the standard parts using this standard. Each part also includes a "courtyard" layer which defines how much space needs to be left around the component for manufacturing; useful when you are designing high density boards.

The standard itself consists of three versions but for most boards it is recommended that the "N" suffix (Nominal) footprints are used. Note that most parts do differ slightly from manufacturer recommended patterns but in my experience (and the experiences of my board loader these footprints do tend to meet manufacturing requirements very well.

As for you query about the 0805 resistor vs 0805 capacitor, I suspect that the footprints have been designed to best attach the parts; while they are fairly similar in the horizontal plane, capacitors do tend to be slightly higher so maybe the differences in the footprints take this into account.