I've got an EVGA G3 1000Watt PSU, with an EVGA RTX 2070 XC Ultra. I'm currently using an EVGA Powerlink with two separate VGA cables coming from the PSU. Would using one of the 8+6(+2) cables be just as effective vs the two separate VGA cables with this PSU/GPU setup? From what I can tell, max power from the GPU is about 240 watts as measured by GPUID. 

Using two cables is better than an single cable, certainly when doing heavy overclocking. When used at normal frequencies a difference between the two would be very slightly and not noticeable in day to day usage. That said, we recommend dual cables as you already have it configured that way.

8-pin Y cables and their connectors that come with your EVGA, as well as other PSUs, are supposed to be rated for more than the allowed load, and I'm sure they are.
 
8-pin PCIe connectors have three +12V terminals..Each single terminal is rated to carry up to 8 Amps of current (96 Watts @ 12V) using Standard terminals for a total of 288-watts, per connector.

Each HCS (High Current System) terminal is rated to carry up to 11 Amps of current (132 Watts @ 12V) for a total of 396 Watts, per connector.
 
Each Plus HCS terminal is rated to carry up to 12 Amps of current (144 Watts @ 12V) for a total of 432 Watts, per connector.
 
The terminals on the PSU itself should be rated similarly.
 
PCI-SIG specs dictate that the graphics card not be allowed to draw more than 150 Watts (12.5 Amps @ 12V) from a an 8-pin (or 6+2 pin) PCIe connector and 75 Watts (6.25 Amps @ 12V) from a 6-pin PCIe connector..This is done by the card's VBIOS and voltage regulation/monitoring circuitry.

PCI-SIG is the group that writes the industry-standard specifications for all things PCI/PCIe that manufacturers should adhere to, just like Intel writes the industry-standard specifications for PSUs that manufacturers should adhere to.

It used to be an issue when Intel specified that PSUs have mandatory multiple +12V rails for safety reasons (Burning Down the House)..Then, graphics card manufacturers complained that Intel's +12V multi-rail spec didn't meet the needs of their more powerful graphics cards and would sometimes trip the OCP (Over Current Protection) on multi-rail PSUs and balancing rails became neccessary..So then, single-rail +12V were born due to backlash from graphics card manufacturers that demanded that Intel's multi-rail spec be voluntarily abandoned to meet their power needs.

See the PCI Express Auxiliary Graphics Power Connectors article over at Tom's Hardware for excellent information that is rather dated, but has not changed that I know of.
 
Go with the single Y cable if you want to..Many people do for aesthetic reasons, as well as miners/people with multiple graphics cards who don't have enough PSU outputs to run an individual cable to each card's PCIe power connector.
 
Class dismissed.