I agree, you are having a gas pressure drop when the table burners are on.
Question: when was the last time the lp tank was filled and what is the liquid level in the tank now? The reason I asked, is if the tank was over filled there may not be enough head space for vapor to form. Thus, the two burners are using ALL the vapor as fast (or faster) as it make. You did was say the table burners are hotter after the new burner installed. This shows the burners were also starved for gas. As the liquid level drops the problem will go away. So you could just blow-off a little gas to lower the liquid level.
A good check for this problem would be to hook up a lp tank that is 1/2 full. Then see if the problem still exist.
1. Try reducing the pressure at the tank regulator to slow down the gas draw from the tank. Pressure is too high.
2. You indicated the new regulator has a y-connection. Install a 2nd regulator on the line to the table burners and reduce the pressure to them. This will make more gas available to the HW heater. This should solve your problem assuming the LP tank is also the correct size..
3. If there is (are) secondary equipment regulators, check them. you may need to reduce their pressure. (???) You don't need item 2 above if these exist.
4. What size is you LP tank? I'm assuming a 20lb tank. Install a second LP tank, both tied together though the same regulator. This will increase the available vapor. ... A 20lb cylinder @ 60 deg F supplies about 24000 BTU. What is the total burning BTU of the 3 burners? I think you could be exceeding the vapor capacity of one tank. Also, as the weather gets colder this problem will increase as vapor supply will drop.
*** If the regulator(s) are set at the proper pressure and the LP cylinder is not over filled, then you need a 2nd LP cylinder (item 4). ****
<message edited by edwmax on Wed, 10/3/12 8:13 AM>