High dollar, overpriced spark plugs are designed, made and marketed for high milage, not performance. Especially when you're talking about a simple little 2 or 4 stroke bicycle engine.
In other words, a spark is a spark is a spark. If you plug fires that's all that is needed to ignite the fuel air charge in the cylinder.
Spark plug gap does play into it. We recommend .024 to .028 as a good rule of thumb to start with. What is more important is the high voltage connection from the plug wire to the plug. The kit spark plug boot could be part of your problem. Replace it with a good automotive grade boot and assure the electrical connection is good. You'll need a new plug, one with the top cap installed so you're not relying on the small threads at the top of your plug to make a good connection.
What is your fuel/oil ratio? If you're mixing at 16:1 that's a bit oil heavy and can impact starting. 24 or even 32:1 is a better option.
Using an occasional squirt of starting fluid won't hurt but do you really want to carry a spray can around with you all the time?
Others will have more ideas and opinions.
Tom