In response to your questions the answer to both would be: it depends.
1. It depends on why you're moving, the economic climate, their pay scales, your USP, and how your target company is performing. In good times, there's little incentive to move for less than 15-20% of your current (base) salary. However, if you're looking to gain additional skills, training, exposure to different types of work or clients then you need to weigh up what you value more - the experience or the pay. You need to be realistic though, a sideways move can be a good thing and open doors, but it may take a while to get back on the same salary trajectory that you may have enjoyed by staying with your existing firm. Frankly after 2.5 years it's difficult to expect that experience to command a significant premium.
2. Mid grade shooting for the next level may get you that salary increment, but it would be quite a risk for you and for the hiring firm. Without the experience you will get found out very quickly, most likely at interview stage. It isn't credible to expect to perform at the grade above without the experience.